I would say that your best bet is to use either FFmpeg or VLC. Both are open source software and are widely used among hobbyists and also as a part of multiple different services. Also both can take in RTSP streams and create an HTTP stream (with live transcoding if needed).
FFmpeg's RTSP support has improved as of lately, but IMO VLC is way better at the moment (May 2010). Check the documentation on the web sites for experimenting with the command-line parameters for finding the required ones to match your needs. FFmpeg and VLC are very flexible and you can do a whole lot of stuff with them in addition to proxying from RTSP stream to HTTP.
FFmpeg: http://www.ffmpeg.org/
VLC: http://www.videolan.org/vlc/