Until Hauppauge released the HD-PVR this past May, there was no way to record in high-definition from your cable or satellite box. Your HD recording options were limited to over-the-air, unencrypted QAM, or, in rare instances, unencrypted firewire from the cable box. In any case there was no way to record the good stuff in HD. I'll spare the details about the HD-PVR itself, and instead include some links at the end.
Of course, Linux support for the HD-PVR wasn't available straight away. However, this is a big breakthrough for homebrew PVRs so the MythTV/Linux community has been hard at work. It there is a Linux driver for the HD-PVR, but you have to check it out and build it yourself (See links at the end). Also, early work has been done on MythTV to support the HD-PVR, but in order to get that support, you have to run MythTV from trunk, rather than using your distribution's packaged version. This means checking out source and building it yourself. The source code changes daily, and at times things get worse before they get better. Whatever you check out on any given day should be in decent shape, but it may not be. You are forging MythTV from the molten center of the Earth, and it's still hot and a little bit on fire.
What is now in trunk will eventually make it into your distribution, but there's generally a 6-12 month lag, depending on your distribution's release cycle. Well, I'm ready to give the HD-PVR a try now, so I decided to give trunk a go. I set up a spare machine on my work bench to be a frontend and backend. After getting the HD-PVR drivers compiled and working, I checked out MythTV from subversion and built it. There are good documents detailing how to do this. It's actually not that hard. The build system for MythTV is very nice. I recommend starting with a MythTV based distribution like Mythbuntu, and removing all the mythtv packages. This will save you a lot of grunt work like setting up MySQL, the mythtv user, init script, etc.
Anyway, I've got the HD-PVR working with MythTV. I haven't used it for any actual TV watching or recording yet. As soon as I can get a frontend set up, I'll be able to actually try using it day-to-day.