One of the great things about Arduino for advanced hackers is the wide array of clones and derivatives. On the other hand, I think the Ethernet and wireless shields are way too expensive. That's why I've chosen RFM12B, for my wireless nodes and R-Pi base station.
The JeeNode is an Arduino derivative, with RFM12B on board, available here in the USA from Modern Devices. I've got a couple JeeNodes and they're great. Jean-Claude Wippler's JeeLabs has extensive libraries, code, blogs and forums. The JeeNode fits (barely) into a Kill-A-Watt P4400, and I'll be posting more about that project soon.
And I recently came across Felix Rusu's LowPowerLab. Felix has created his own Moteino, another Arduino-with-RFM12B derivative. I just got two of them and they look really good:
I'm really pleased with the design and build quality on the Moteino, and looking forward to using them, and blogging about them. As you can see, they're very compact, with the radio on one side and the ATMega328 on the other. One of these will easily fit in a KAW, and in fact Felix has posted his own WattMote project.
Felix and I traded some notes on KAW vagaries, and agree that the biggest hurdle is getting enough power out of the KAW to run the radio. Which is somewhat ironic considering it plugs into the AC mains. But we've each got some low-power tricks to share, so check out his project and stay tuned here for more details on mine.
Next up, my circuit analysis of the KAW.