The network thermostat that I mentioned previously is now for sale at Smarthome.com. It for sale for $399, which is too expensive for me. Also it looks like this requires a CAT5 line run to the the thermostat, in addition to the HVAC control lines.
It is surprising that they would include the punch down block to include Power Over Ethernet. I would assume that all they would need to do is have a little box that had two ethernet jacks, and a jack for the power adapter. (This is what was done when I was using wireless broadband.)
I also wish this had 802.11 so it wouldn't be necessary to run and additional CAT5, but I don't know if it is possile to power a 802.11 device from the 12 volts DC running on the HVAC wires.