CyberLink's PowerCinema is one of the new software options for Linux-minded entertainment-PC builders. The company pitches the Linux version of its software as an "easy means of converting the Linux PC into a feature-rich TV entertainment center." In more practical terms that means it supports TV tuner cards and provides an EPG (Electronic Program Guide) for setting up recordings. Right now it's available to OEM system builders only, but hopefully that will change soon.
Another Linux option is SageTV. The company recently decided to make the Linux version of its DVR software available to everyone, not just OEMs. Check out our CES 2006 coverage for more details.
If you're looking for a step-by-step guide to putting it all together, Linux.com posted an article earlier this month called, appropriately enough, "Building a Linux home media center."