Ovbiously, I'm a little late for this one, but I have to plug my favorite ID3 batch tagger. For the past year and a quarter, I've been retagging all my MP3s with:

Tag&Rename

I'll swear it's the most powerful ID3 tagger and renamer (because it's the only one I've ever used). If you can think in terms of parsing and variable substitution, you can build VERY powerful commands to retag or rename your MP3s very easily. You should see some of the tricks I've made Tag&Rename do.

Ok, enough advertising. Just check it out.