There are two kinds of USB TV available to my knowledge. Both receive analogue broadcasts, but one (as mentioned above) sends an MPEG stream to the host PC instead of raw frames. The one sending raw frames has very poor picture quality due to the limited bandwidth of USB 1.1 (all that was available when it was released) and the one sending MPEG requires the host PC to decode the data before displaying. In both cases, the computer's processor is put under load to display the image data on screen. Due to this, I would say that writing drivers for USB TV receivers solely for watching TV is pointless as the performance would be disappointing, but the task not impossible.
With a PCI TV card, the card itself writes the image data into screen memory, putting zero load on the processor.
I have the Plextor 402U here but I haven't started writing a driver for it yet. It would certainly be a great video acquisition tool for RISC OS.