[Brian]

100% possible that I'm wrong, but I don't think this is unique to OmniFocus. I use MobileMe for my personal email, and for a few days after updating to 10.6.5 and 4.2 on my various devices, they were constantly asking me to re-enter my MobileMe username and password.

It annoyed the heck out of me, but I just kept re-entering my info every time I tried to send mail, etc. Finally settled down sometime over the weekend.

My completely-without-evidence suspicion is that they made some changes on the back end to support the new login form they rolled out on the MobileMe website. Presumably, that was because adding the free "Find my iPhone/iPad" feature no longer meant that everyone logging into MobileMe had a .me or .mac email address...