This is perfectly possible and it is how I have replaced the Apple TV units we had used up until Christmas.
TThough but before we get started WiFi cannot be used due to the bandwidth requirements of Freesat HD and 1080p content.
Using MythTV + DVB-S card as the backend server, picking the right card is crucial BTW. around £50 ea
Then use the XBMC Live installed on a Acer Revo Nettop as the streamer.
Acer Revo £148 + £10 for Cyberlink remote control.
XBMC has a little known feature of allowing direct connection to both the recorded programs and the live TV


The MythTV host could be used as a frontend as well but I doubt 1080p is possible within the MythTV frontend.
R