I tried everything I could think of to get .Mac sync feature to work following an upgrade to 10.4 ... but nothing seemed to work.
Finally, I found this article that informed me that there can no longer be an ampersand (&) in the user name. Changing the ampersand to "and" fixed it, and I was able to sync all my computers with .Mac. Maybe all of you power users had already figured it out, but I hadn't seen this hint anywhere else.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2005050611345322