For anyone here who makes use of Netflix streaming, they have seasons 1 through 5 available in HD. That's how I managed to fly through all 5 seasons in a few months prior to season 6.
I used my PS3 to watch, and I was pretty impressed by the image quality. Not as good as Blu-ray, but better than the overcompressed broadcast HD I've seen on Comcast. If you have the bandwidth and a Netflix-capable device (networked Blu-ray player, 360, PS3, etc.), it's definitely worth checking out.