Encoding format of choice?

All my CD's are encoded to MP3's. Should I stick with this format or re-rip them all to OggVorbis for my Karma?

I don't have the disk space to rip them all to FLAC

I'd say recode only if you're unhappy with the quality of the mp3s or you're running out of space and you think that quality equivalent Ogg Vorbis files are smaller. There's no reason you couldn't start ripping new things in OV if you wanted, though.

If battery life on the Karma is important to you, leave them as MP3. The MP3 format consumes the least amount of power at playback time.