Make sure your permissions are set to "Read & Write". Just do a Get Info on the MP3 in finder and then change it. Hope that helps, I had the same problem a couple of days ago.
This happened frequently on OS X.1. After I upgraded to Panther the "feature" seems to have disappeared. And I've done lots of renaming (I'm ripping all my vinyls and CDs to AAC), both directly in the main window and by using Get Info.