After upgrading to iOS 4.2.1, OmniFocus wouldn't allow me to sync to my iDisk. I should have copied the full error message down, but it was along the lines of:
WebDav sync error 500
I found this older thread when searching:
http://forums.omnigroup.com/showthread.php?t=10820
and by changing to a different server, getting a password error, and switching back to MobileMe everything was corrected.
It seems like this is a not uncommon occurrence when upgrading firmware. Perhaps it would be worth considering adding some button that lets you reset/re-enter your password under the "Sync Settings" pane. Do folks encounter a similar problem when changing their MobileMe password?