I think DerekWeb got it right... You have options to choose from in the drop down list of formats when you download. One option is HTML (excellent for using iSiloX to convert to iSilo format). Another is rtf (which I picked because it's good for converting to eReader/PalmDoc format). The one you may have been looking at is the cross platform, which does indeed seem to be MobiPocket.
But all the discussion got me interested. It's on my "might read if it looks interesting when I'm picking a book later" list.