If you're asking if the N9 can play a 720p video without it being converted beforehand, then I believe erendorn had the answer earlier in this thread:

"N9 currently supports H.264 Baseline profile. It doesn't support Main and High profile yet (and maybe never). I don't know if it supports other codecs (or will)."

So as long as your video (720p) is using the H.264 baseline profile then yes it should play ok. I say "should" as I don't have mine to test it myself yet!