Most everything is M4V it seems these days (and I've found it's a more consistent quality), or you can convert it with Handbrake if you happen upon something like that.
Yea, I fully support M4V for their consistency, but I have a ton of old AVI files I would like to stream through Apple TV that won't merely because they're AVI. Those could be converted, but old stuff like some TV series that I only have in AVI are screwed because they would take forever. Just seems like an easy fix.
Also, completely agree on Rdio - one of the best music services out there. No programs needed. Beautiful design.