Hmm. afaik xine has some working rtsp:// supoprt for the realmedia streams. dunno how does it work, but last tiem i checked the code it was quite huge (many files, parsers, crypto stuff etc) compared to the pnm:// code.
Yes, the Xine folks have shown that some RealAudio/RealVideo "rtsp://" streams can be played. I hope to someday reimplement this as part of the LIVE.COM Streaming Media Code - which will thus make it available for use by MPlayer. The particular problem that Francisco Leon noted (with "rtsp://start.real.com/..." URLs) was different, because (i) that particular server behaves in a weird, non-standard way (it's not one of RealNetworks' 'Helix' servers), and (ii) those streams require a paid subscription anyway. Ross.