Re: But why?
Yep, this is purely a US thing, but it's a pretty big market.
While TV tuners have been around for years, the first CableCard tuner only turned up a little over 2 years ago - the Ceton InfiniTV has 4 tuners, and when installed in a Windows7 machine with Windows Media Center allows the user to record up to 4 shows at a time from cable, including "premium" channels, such as HBO and ShowTime.
Even at the initial price of $400, paying $3/month for a cable card instead of a $20 for the cable companies DVR made it a pretty popular option - there was a waiting list for at least the first 6 months after the device was finally available. By now, the price is down to $150 on sale, and SiliconDust and Hauppage also have Cablecard tuners available, and Ceton are promising new extenders by the end of the year.
Because most Cable providers encrypt most of their content, you can't use the Tuners with MythTV or Sage - because Microsofts PlayReady DRM has been clears by CableLabs. MediaCenter gets access to the content, and re-encrypts it on disk as required.
At least in part due to the popularity of these Tuners, in the last 12 months the FCC has required that the Cable Companies support User-Installed cable cards (no mandatory $90 "installation fee" for that $3 cable card, which was very common for the early adopters).
An AppleTV box with a Tuner and a 1TB hard drive (not much point having a DVR with less than that) will probably need to retail for $300-$350. With access to iOS apps and easy "screencasting" to iDevices, it could be a very popular option. Replace a single cable-company DVR and you could pay for this within 18-months to 2 years, with additional functionality.