I am having this problem too, ever since I did a clean re-install of OSX. I'd be perfectly content to erase _everything_ on .Mac and start over, but unregistering my iMac, deleting everything that I can reach via the browser (contacts, bookmarks, and mail) doesn't help with sync refusing to accept the correct password to sync the keychain.
I found a .plist file that may be relevant. It's
/Volumes/iDisk/Library/Keychains/.syncinfo/login.keychain.syncinfo.plist
The file has the date that I bought the iMac, not the date that I reinstalled the OS. I can't delete the file, even with sudo rm, although I'm both the file owner and group. Any ideas?
If it's a likely solution, how can I delete everything on .Mac and start over with what's on my iMac?
-- A recent & happy switcher
(If only I had escaped MS-world years ago!)