If you have a search around there are people posting on this subject.
A couple of companies/products I've seen mentioned are
www.kat5.tv and
www.avnex.co.uk
I'm not sure if you're talking about using a Media Centre PC or having a 'media centre' as in a central hub with all of your boxes?
If it's the PC option running Windows Media Centre then you can get Extenders which allow you to access content on the server via a standard ethernet network. These are available in various guises, dedicated boxes, an 'expansion pack' for the X Box (the new X Box 360 is supplied as an extender) etc...
I'm looking into multi-room, multi-source with a server for music and movies etc...
Another product to look at is Sonos. They onyl support audio at the moment but I'm hoping they might move to video in the future.
I must confess although there's alot of info on this site people seem very reluctant to post......I've had items on here for days with no response!?!?!?