I don't know about Flixter. I think it can only be viewed via VUDU and UV's website. I know I can't access it with CinemaNow.
If you can access movies you get via InstaWatch on another service then that should mean they are UltraViolet. Thanks. Sorry for any confusion by the way, just because a movie is owned on Vudu doesn't mean that it is a UV copy (I have about 5-10 movies that are only visible on Vudu) so I've been wondering about that.