Each Tweet-a-Watt device contains some power monitoring electronics and an Xbee module. The Xbee modules relay energy usage data for each Tweet-a-Watt back to a single receiver, which collects the data.
In staying with the overall green theme of the project, Ladyada used an Asus Wl-520gU wireless router to control the receiver and collect data. This is the same router I used for my Wifi Radio project! The WL-520gU consumes ~5W and has enough horsepower to run the python data acquisition scripts while still acting as a cablemodem gateway.