I've just finished putting together a dual boot htpc. I knew in advance that mythtv/mythbuntu wouldn't play bluray or HD disc. Also I had a major life lesson when I found out that everything had to be HDCP compatible. Everything! insert Buy new tv here. even though the sales guy said this one is HD ready. Either he didn't know his product or I was a gullible idiot that really should have asked him to play a bluray or HD disc to prove it would do what I wanted it to do 16 months later saving me a 3k upgrade though I do have a really big monitor now. Anyway off track a little..
The HDCP thing, everything from the where the disc is played to where the audio and video end up has to be hdcp compliant. I chose the XFX geforce 9300 motherboard from playtech. I had read good things about it and that it was HDCP compliant. I never managed to play a bluray movie though and in the end returned the board. No amount of up to date drivers, bios, clean installs or differant software players made any differance. My exact problem was that the signal going to the tv was digital but not HDCP compliant.
http://www.flickr.com/photos/blury-tests/ for a screen shot.
I'm now looking at the P5N7A-VM board to replace the xfx one. Apparently a good test is will it play sony bluray disc as sony tends to be anal about the whole you must be with us or against us thing. I'll still be looking at using mythtv for my recordings and power ultra to play bluray movies.