FBReader clearly needs more support from programmers. They have been concentrating on the iphone for months, unfortunately for us.
I am trying to convert a big (17 MB) DRM-free mobi file to epub in Calibre...24 hours and it still is not ready (on a Core2Duo). So clearly FBReader is the way to go. Especially since I want to be able to use much larger files, those huge Wikipedia files: