If you have Privoxy installed, you'll have trouble mounting your iDisk. To get around this, go to the Proxies tab in Network Preferences and enter "mac.com" in the bypass list.
[Editor's note: I haven't tested this myself, and an earlier hint seems to imply that the bypass setting doesn't work properly. Can anyone using .mac and Privoxy verify this hint?]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021122054435771