Sorry for posting a link to a Dutch webpage, but it shows 2 images that pretty much speak for itself. It shows a Dell laptop that went into spontaneous self combustion mode. Note that these laptops use the same Li-ion battery type used on the MER rovers. Well, personally I think going out with a big KABOOM is a more appropriate ending to such a great mission than just fading away. I'm not implying that this will happen of course... just mentioning the possibility.