This has probably already been asked before, but will the riocar software ever be updated to support OGG Vorbis format files?

Or does it already support them? I've never actually tried playing one on a rio car before.

This one's in the FAQ, but here's the quick answer:

It does not support OGG right now. There is a possibility that it might show up in a future software release. Not any of the bugfix releases coming up in the near future, but rather a more distant future release.

Give the FAQ a read-through when you get the chance.