Every now and then someone seems to post something about losing templates/widgets or having them corrupted during an upgrade. If you have had this sort of thing, it might be helpful to 6A if you could write up an explanation of what happened to you including details like the version you came from, the version you went to, what plugins you had, and what templates you were using (did you customize them?)
Reported on Movable Type 4.2
Mike - I had an interesting upgrade experience this is sort of related to this question... posted about here:
http://forums.movabletype.org/2009/04/425-upgrade-database-strange-behavior.html