I recently switched to Entourage for mail (I got corrupted Mail files, and couldn't clean it up, but that's another story). Anyway, It was asking for my Keychain password over and over, about 30 times a day.
It turns out that I had two keychain files. Run Keychain Access (in Applications -> Utilities) and it will show all keychains. Delete one, and Keychain will only ask once.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2003082822315680