You can actually use the MediaPlayer
player = Manager.createPlayer(url);
videoControl = (VideoControl)player.getControl("VideoControl");
Field videoField = (Field)videoControl.initDisplayMode(VideoControl.USE_GUI_PRIMITIVE, "net.rim.device.api.ui.Field");
videoControl.setDisplaySize(XXX , XXX);
Or else you can use a BrowserSession and it will then invoke the Standard Media Player and play the RTSP stream...
I have to mention that you will have to make sure that your firewall is set up properly to allow the RTSP stream through, it took me a while to figure it out and the Blcakberry is a bit fussy about stream playback.