I doubt you'd be able to stream either of the HD formats over the network because of the studios' requirement that all your hardware be HDCP-compliant. In theory I suppose you could preserve the encryption of the content over the network and decrypt it at the display device, but I don't think the standard is designed for that. Hell, one person here reported that, by design, his HP laptop couldn't even display a Blu-Ray disk on a compliant external TV set; he could only watch it on the laptop's screen.
Material encoded in DivX or XviD is an entirely different matter. I routinely watch XviD/AVI and H.264/MKV content over a network. I just have the player software (mplayer in my case) open a remote file on the server. 802.11g wireless or 100BaseT ethernet has more than enough bandwidth even for 720p, and probably for 1080p, content. (I'd have to do the math for 1080p, and it's too late at night for that
You might be able to cobble together a solution if you can export the desktop of the machine playing the disk to the remote client with something like Remote Desktop, VNC, or X Windows. Basically you'd be displaying the material on the server and just shipping the display across the network, so HDCP-compliance might be maintained. I'm just guessing about this one, though.