I just spent the weekend setting up MythTV on my ps3 in Gentoo Linux. So far, it's working quite well. It will only run in framebuffer however. I haven't emerged lirc yet, because I don't have a remote; just using the keyboard. Anyway, using samba, I was able to setup my network shares from my Debian server, and added the paths to all my xvid movies and music to the MythTV frontend. I set MythTV to use mplayer as the default, and it plays all my xvids just fine. I had to tweak the mplayer settings to get the best performance I could, and streaming the videos over my network is *slightly* choppy, but quite good. I guess this is due to the fact that we don't have access to the ps3 GPU yet. I'm still playing with settings, but all in all, it runs very well.
I decided to install fluxbox instead of KDE or Gnome, because of how lightweight it is. Still, when streaming videos over the network, mplayer and X use about 70% of the CPU.
I have everything setup so that once Gentoo boots, it logs in to fluxbox, and the MythTV frontend is automatically started.