A couple HDTV tuner cards have open-source driver support in Linux. Look up pcHDTV 3000 card, I can't remember the other cards names, but they all seem to be supported by the Linux DVB drivers. I don't know if the PPC linuxes are up to using these, especially as these drivers are now part of the 2.6.12+ kernel source tree, but if you want to fight your way through the Linux DMA issues with it (I'd imagine you'd want DMA to dump direct from tuner to hard disk), let us all know how it turns out.
I'm also not sure if an 800MHz CPU would be up to playing back the saved data, but saving it __may__ be possible as this part is only a direct copy of MPEG data from tuner to hard disk with no encoding in software to do. Decoding for playback is quite demanding and I'm told _barely_ works on 1920x1080i with an AthlonXP 2400+, so don't hope too much for it to work well on 800MHz of any CPU.
