Somebody we know around these parts has posted a nice tutorial over at MacDevCenter on how to use VLC to stream live HDTV from an EyeTV compatible tuner to other Macs around your house. This way if you set up a Mac media center computer to record HDTV, you can shoot that content to other Macs (or even, perish the thought, Windows PCs, since VLC is cross-platform).
The same basic methodology should allow you to stream already downloaded/recorded programs as well. Since VLC has a web-based remote, you can even control it from the client computer as well. There are many good non-obvious tips here, particularly Erica's way of exposing the MPEG transport stream, so if you're interested in a Mac based HDTV system, check it out.