If you are having trouble logging into your previously created Apple account through the iTunes music store, and you have a long password, you need to change your password.
The input field in iTunes only allows a certain number of characters, even though when you create an Apple ID you can have a password of unlimited length. I filed this bug with Apple.
[robg adds: I haven't verified this one; if you have any experience with the problem, please post your comments.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030429034120194