Over the past few weeks, Media Portal, the open-source HTPC software gained support of the Hauppauge HD-PVR including dual-HD-PVR support. Media Portal joins the ranks of SageTV, BeyondTV, GBPVR and MythTV. I've heard from an insider that Mac's Elgato HTPC software will have official HD-PVR support very soon as well.
Of the main HTPC competitors, that leaves only Microsoft Media Center without any hope of HD-PVR support within the coming year. I would expect that once Windows 7 is released, HD-PVR support would be possible since h.264 will be a supported format as long as Microsoft allows h.264 to be one of the supported recording formats. The problem is Windows 7 won't be available to the public until September 2009 at the earliest.
If you're interested in using the Hauppauge HD-PVR to get liveTV and recorded TV in all of its digital and HD glory, here are your options:
SageTV XP, Vista, WHS, Linux, Mac and HD100 extenders
BeyondTV with XP and VistaGBPVR with XP and Vista and PopCorn Hour
Media Portal RC3 with XP and Vista (download & further info)
Elegato Mac (coming soon)
Read more about the Hauppauge HD-PVR