I just went through the trouble of setting up a audio/video RTSP stream and did get it to work on my Storm2.
Biggest problem I had was the BB is picky on the codec used inside the stream, and gives very little info about why it doesn't like the stream when it does have problems.
I found a list of supported media types on the BB: Media types that are supported on the BlackBerry smartphone
Biggest gotcha I can see: The BB will play mp3's locally but can't play them via RTSP.
Do you know what the data format the stream uses? Sadly, the stream itself might have to be changed to get it to work.