A new paper in press with Icarus:

A distant planetary-mass solar companion may have produced distant detached objects
Icarus, In Press, Corrected Proof, Available online 26 July 2006
Rodney S. Gomes, John J. Matese and Jack J. Lissauer
Abstract
Preprint (1.67 Mb PDF)

There is also a pretty good Wikipedia page on scattered disk objects.