Most homes have coax wired into almost every room, but only newer ones have Ethernet wall ports. D-Link's DXN-221 Ethernet adapter works around this problem and comes with two units, both with a Coax F-Type connector to go into your wall, and an Ethernet adapter to hook up to your networking on either side. The adapter works in the 800-1500MHz range, which supposedly doesn't interfere with your TV signal. All this can be yours in Q3 2008 for $199 for two, and $109 for individual adapters thereafter.
D-Link doesn't have images of this thing, so I made my own interpretation of what it looks like. We've got actual pictures! See my original interpretation below. [D-Link]