Regarding mp3 files, the mp3 codec in patented, but ID3 is not. Though it's OK to read ID3 tags from mp3 files, it's better if you entirely drop support for mp3 in your tool.
Do not take it otherwise, but I would wait for RMS' opinion on this issue. There are a few GNU packages which flaunt MP3 support too.
I know what RMS is going to tell you.
Seriously? Then why dont you do the favour of letting me know?
Regards, Debarshi