Jeremy, shamelessly pimping for his work has suggested using a PS3 as my MythTV front-end as an alternative to building my own front-end. Advantages being it could be slightly cheaper (depends on hardware choices), Blu-Ray drive thrown in, and it's got some pretty awesome hardware. I'd add that I'd get a PS3 as a significant advantage.
It's a very tempting idea, and one I'll think about quite seriously. Current MythTV support seems a bit patchy, but it also seems to be improving pretty quickly. Games consoles are a natural platform for this kind of hardware: it's already designed for the lounge room, it's got all the hardware you need already, and geeks wouldn't complain about a shiny new games console in the lounge room.
I might see how I go with SD and the new telly for a while and keep an eye on progress of MythTV with the PS3. If I decide I want HD, it's a very good option.