[Edit] None of this is probably new to you guys, but I thought I'd share my thoughts anyhoo!
Using a generic USB DVB TV card I've now got the PS3 showing Freeview (the digital terrestrial solution in the UK), with the ability to record.
Now I'm no guru, so this is how I'm running it and later I'll crib some notes together from the various howto's I used.
Ubuntu picked up my USB DVB stick automatically (which I was VERY surprised about) - other sticks may vary drastically.
The app I used to start off with was Kaffeine, which looks like a great app as it is totally GUI and didn't require any command line config. However, the video playback was very jerky. BUT, if I recorded live tv and loaded that file in VLC it played flawlessly (although with maybe the audio sync out a few milliseconds).
I have also installed both the MythTV front and back end on the PS3. This gives about 1 frame a second and the UI was a bit sluggish, but it 'worked'. I can't say how recordings play back as I haven't tried that yet. Also, MythTV is VERY configurable, so this could possibly be improved by someone who knows their stuff.
The best performer by far is mplayer, which plays at (at a guess) ~20 frames a second. This varies though - a panning shot in a city causes a fair bit of stuttering, but watching something largely static or slow (ie BBC News 24) was excellent. The only downside with this is that you're forced to use the command line to change channels.  Also, sound begins to 'pop' after about half an hour - exiting mplayer and rerunning the command fixes this.
All of the above were installed using the distributions provided to Ubuntu through apt-get.
OK, so now for your help. From what I have seen, VLC provides exceptional playback. Is anyone aware of a TV app that can use VLC as a player? Or do you know of any tweaks to the above systems that may increase performance? The most promising app so far is Kaffeine - I think MythTV may just be too big. Ubuntu was installed with the PS3 beta (to get around the 15% install bug) and updated, but hasn't been tweaked.
Anyway, thats what I've got out of a days investigation. If ANY of the above is incorrect or misleading, please reply to this thread and I'll update this post accordingly. I'll also write up how I put this together when I get a sec.
I was very pleasantly surprised by how well the PS3 handled live TV given that Linux support is 'new' - I'm seeing great things for the PS3 as a PVR.