I just upgraded my machine at work to OS X 10.6 (Snow Leopard). It is one of 4 machines that I regularly work on. I keep the passwords and PIM data on these machines synced using MobileMe. Upon the first run of the MobileMe sync process after the upgrade, I got this dialog:
So apparently I just need to guess.
I just struggled thru this recently. It wanted a password from a machine I don’t own anymore. I totally reset my mobile me syncing, and still received this message.
However I did find a fix. In the Keychain preferences, under General, is button called Reset Keychain syncing. It no longer bugs me about putting in a password from 4 years ago.
Big ups, Nerdifer!