That's a good question, which doesn't actually seem to be in the FAQ, at least not in that pithy a form. You need v3alpha11 if you want support for Vorbis or FLAC files, or for Unicode characters beyond Latin-1 (e.g. Greek or Cyrillic). And possibly for proper gapless MP3 playback too (it's a long time ago, I don't remember how well that worked in v2).

If you don't need any of those features, you might be better off with v2.01, as there have been reports of problems with v3alpha11. On the other hand, some people do use v3alpha11 every day without issues. (Really, alpha 10 should have been called "beta 1" and alpha 11 "beta 2".)

Peter