Burt and Knauth's LPSC abstract on their theory is http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1869.pdf . Burt also has an abstract from his poster on the subject at http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2295.pdf -- including a highly acerbic section on a "young sedimentologist" of his acquaintance who misinterpreted terrestrial basal surges as water-laid tuff. Seems the ingrate didn't even acknowledge in print that Burt had instantly disproved his long-time belief... This debate seems to have the potential to get seriously personally nasty.
While Burt and Knauth object for other reasons to McCollom and Hynek's rival theory that the Meridiani deposits are due to a VOLCANIC -- rather than an impact-caused -- basal surge, they seem to agree with them that one of the biggest objections to the MER team's belief that the deposits were laid down by alternating wet and dry episodes on the surface is that it stretches chemical coincidence. In their own abstract ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2023.pdf ), McCollom & Hynek reiterate their argument at the December AGU meeting -- which touched off quite a spectacular brawl itself, which unfortunately I couldn't hang around for the end of -- that the MER team's theory is Rube Goldbergian.
The MER team thinks that the sulfates in the Meridiani deposits were originally produced ELSEWHERE by exposure of basalt to acidic groundwater, then dried out, then got blown by the winds to mix with the native sands at Meridiani (which were NOT raw basalt, but already consisted of siliciclastic sands which had PREVIOUSLY themselves been basalt exposed to sulfuric acid, but had then had all the sulfate salts which had thus produced locally leached out of them). This mix then got exposed to groundwater AGAIN (this time non-acidic, and/or relatively small in quantity), so that it only modestly redissolved the sulfate salts, which then dried out and recrystallized mixed with the same siliciclastic sands -- the whole mess having been changed in physical texture, but not further chemically altered, by that last exposure to kinder, gentler groundwater.
As McCollom and Hynek point out, it's stretching coincidence in that case to assume that the current Meridiani layers just happen to have precisely the same ratios of different elements that you'd get if they had been made far more simply by just ONE exposure, in-situ, of local basaltic particles (sand or ash) to sulfuric acid. In the MER team's model, you have to mix inblown sulfate powder with in-situ siliciclastic sand in just the right ratio, by pure coincidence, to get that same mixture of elements: "... the bulk composition of the [Meridiani] bedrocks would require that the amounts and relative proportions of cations that were removed from the primary siliciclastic component prior to its incorporation into the rocks must be exactly balanced by those added back in by evaporating solutions, so that the final rock has the composition of Martian basalts. Although not impossible, it seems like a somewhat unlikely coincidence that this would occur and that the balance would be achieved so uniformly in all rocks at Meridiani." (They add that it's hard to think of a source of such massive amounts of pure sulfates elsewhere to get blown into the Meridiani region, since Mars Express hasn't seen any comparably huge deposits of phyllosilicate sand or rock elsewhere that would have been left behind after the sulfate powders had been wind-blown out of that area.)
So McC. and H. instead argue that the Meridiani layers are the result of a Martian version of Mt. Katmai's Valley of 10,000 Smokes, but much bigger and a lot more acidic (which is plausible, given that Mars seems to have far more sulfur in its overall crust than Earth does): "In this scenario, the rocks were initially deposited as a series of volcanic ash flows of basaltic composition. Following deposition, the ash deposits were permeated by SO2- and steam-rich volcanic vapors that altered the ash at elevated temperatures. In this process, SO2 and H2O in the vapors combined to form sulfuric acid, which then reacted with the rocks, similar to the acid-sulfate alteration observed in volcanic environments on Earth. During alteration, the original igneous minerals are replaced by alteration products including phyllosilicates (e.g., nontronite, saponite), amorphous silica, hematite and sulfate salts. Morphological features such as cross and festoon bedding, interpreted by the MER team to result from eolian and fluvial processes, are also observed in base surge deposits in volcanic settings and appear to be consistent with a volcanic scenario. The scale of the deposits appears to be consistent with large volcanic deposits elsewhere on Mars." Burt and Knauth agree, except that they don't think there are signs of any volcanic regions in or near Meridiani big enough to do this.
Now, McC. and H.'s main argument, the chemical one, could seemingly be explained just as well if local basalt sands at Meridiani were simply exposed to highly acidic but cool and liquid local groundwater that gushed into the region from somewhere. And Benton Clark's observation that Martian atmospheric processes (due to the fact that solar UV can reach all the way down to the surface) can generate far more sulfuric acid on that planet's surface than on Earth could explain the existence of such cool acid solutions. After the fight at the AGU, I phoned and E-mailed Hynek on this point, and he actually agreed -- but said that the MER team apparently disagreed with that particular theory on some grounds involving grain texture in the Meridiani rocks, about which he really didn't know the details. Now, another print-only LPSC abstract by the MER team ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1655.pdf ) explains their reason for opposing this version: "Observed compositions require either that basaltic sands were altered in place by abundant and pervasive acidic groundwater, or that alteration occurred primarily in the source region, followed by the formation, transport and deposition of sulfate-rich sand grains, with subsequent diagenetic redistribution of the most highly labile mineralogical components. The roundness of observed grains and, especially, the locally complete obliteration of grains during diagenesis strongly favors models in which alteration precedes sand generation and transport." That is, the MER team thinks that if the layers were produced by the in-situ exposure of basalt sand to sulfuric acid solution, MER-B's Microscopic Imager would be seeing both much sharper-cornered siliciclastic grains in the stuff, and a lot more still-recognizable grains of locally crystallized sulfates that had not been ground up into powder.
I wonder about that, though -- especially since Mikhail Zolotov thinks that sulfuric acid solution reacts very dramatically and efficiently with basalt, perhaps so well that by itself it totally dissolves and remixes the resulting sulfate crystals and also rounds the grains of left-over siliciclastic material from the basalt. Knauth and Burt actually use Zolotov's statement, in their LPSC abstract, to argue against the idea of the Meridiani layers being created by ANY kind of exposure to sulfuric groundwater: "Zolotov  has presented a compelling argument that regional acid aquifers on Mars are untenable because acid would be quickly neutralized by reaction with basaltic material." But if the surface production of really large amounts of sulfuric acid on Mars really is as feasible as Benton Clark thinks -- by atmospheric rather than volcanic processes, out of Mars' surface water in all its different physical phases plus the planet's volcanic sulfur dioxide -- it might very well be possible anyway to get the huge supply of local H2SO4 solution needed to radically modify all that huge supply of basaltic sand that was originally at Meridiani. Knauth and Burt also do seriously question how much the grain textures in the Meridiani layers really can tell us about how they were formed.
In short, there may still be FOUR different models for how the Meridiani layers were formed -- and the simplest one, that local basalt sands got soaked in local sulfuric acid-rich groundwater, may turn out to be the correct one after all. I've really needed to talk to Hynek about this again for some time, and I especially do now.