In some MP3s I have, the ID tags were in all upper case and I wanted to change them so just the first letter was. I noticed that iTunes (7 at least) would not let me change case -- it would complete the word in the case as it already was, and not let me change it. THis was driving nuts, but I did figure out a workaround.
You can paste from the clipboard to change the case, and it will accept it that way. So just type it as you want, copy it, then paste it. Hope this helps.
[robg adds: I thought we had published something like this earlier, but I can't find it...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061119221649950