ReplayTV support still works as of XBMC v10.1, but since ReplayTVs stopped being made in 2005 you should expect support and testing to be very limited.
ReplayTV is a stand-alone PVR/DVR (Personal Video Recorder/Digital Video Recorder) box for the American market and is sold online at www.replaytv.com. As a rival to the famous TiVo, the ReplayTV PVR/DVR box records TV (and schedule recoding) to its built-in hard drive and stores it there. Via the network-port built-in to the ReplayTV box it can share the stored video to a local-network. XBMC has a DVArchive-client which can auto-detect, browse and stream/play such shares.
For auto-discovery (default in XBMC) use the following Media Source in the video section:
<source> <path>rtv://*/</path> </source>