Why would I want that? Not because mp3 is in any way superior, is just my phone doesn't support playing FLAC and its audio capabilities are no match for flac.
If you want to convert FLAC to mp3, say to listen to your music on your phone or mp3 player, and you use GNU/Linux I recommend soundconverter.
This is already packaged in Debian and available in etch, lenny, squeeze and sid. It is probably available on Ubuntu and all other major distros out there, so, enjoy.
(This is also a reminder for me. I used soundconverter before, but I was unable to remember which was the application.)