FYI, thanks to Michael Niedermayer's work on the FFmpeg H.263 decoder,
mplayer - when built with the latest FFmpeg and LIVE.COM libraries, can now
properly play H.263+ RTP streams.
For example, the following example now works:
mplayer -fps 15
rtsp://darwin.horizonlive.com:554/finlayson/sample-h263-ulaw-8000.mov
Ross.