Wrong! For years now MythTV has been arguably the best way of watching media in a dedicated environment. But it's just so damn hard to install and configure that most Linux users don't give it a chance. This vacuum of media centers on our beloved platform is about to get filled however.
The 1st option for a Linux media center is still MythTV, I heard you roll your eyes! The difference is that this version of MythTV comes with a buntu suffix. Mythbuntu is a tailor made ubuntu variant which is built from the ground up to support MythTV's ever growing list of features. And although it doesn't run as a liveCD if your building a dedicated media machine your not going to mind too much.
However if your not quite ready to replace your current Linux install with a media center and you don't want to go down the dual booting root then the next 2 options are right up your street.
The XBMC project started out as a hack for the original Xbox to turn it into a quiet and (relatively) small media center PC. The project since, however, has grown exponentially and now supports Windows, Mac, Linux, Xbox (original) and even the Apple TV. Thanks to its roots the project has a number of very good hardware and software hackers in its community and has enough skins (themes) and applications to rival MythTV.
A spin off project from XBMC comes in the form of Boxee which is a sort of social media center allowing users to rate, comment on and suggest films to other users. This social element is carried on with intergration into Twitter as well as integration with Last.fm, Flikr and for all you luck Americans Hulu! Yep free streaming movies and TV through your media center! You get all this AND the software is still only in private alpha. However you can register to take part in the alpha at the address below. For an alpha it is incredibly stable, probably because of its XBMC roots so there should be few worries over hardware compatibility.
I personally use Boxee as the user interface is very slick and the social watching element appeals to me. So if your also a Boxee user my username is, as ever, grindboy so please feel free to add me.