But there's a really simple solution: OS X 10.5's Screen Sharing, which works flawlessly and 'streams' all of my Mac's contents. The only pity is that it doesn't send the audio as well. Enter Soundfly -- it streams audio in good quality (various settings available). Now with Screen Sharing and Soundfly running, I can watch live TV and movies downstairs. Because I also can remotely control the Mac upstairs, it's also possible to start and stop recordings, and more. This works quite well.
Note: As far as I can tell, Soundfly simply redirects the audio from one Mac to the other -- the sending Mac is then mute. If you want audio on both Macs, install Soundflower, which gives you the ability to hear the audio on both Macs.
[robg adds: I haven't marked this one 10.5 only, even though Screen Sharing is a 10.5-only feature. Pre-10.5 users should be able to do the same with the various VNC server/client solutions out there (i.e. Vine Server, Chicken of the VNC, Jolly's Fast VNC, etc. I haven't, however, tested any of those solutions.]

