After reformatting my Macbook Pro and re-adding my MobileMe account, I found that my Calender wouldn't sync. It repeatedly asks me if I want to Merge the data or replace it on either the Mac or MobileMe. Regardless of what I answer, it will silently fail and pop up the same dialog again in a few minutes. I've tried unregistering my Macbook Pro (as well as the pre-reformatted Macbook Pro) from MobileMe and electing to reset the sync data from the newly formatted, clean iCal. Same thing, a silent fail. My mac pro at work still seems to sync with it, as does my iPhone. Also, since yesterday I've noticed that Mail.app somehow forgets the passwords to my two email accounts. So, I'm thinking that there may be some kind of issue with syncing Keychains as well?