Tokyo-based wireless hacker Akiba of FreakLabs sends word of their latest project, the Freakduino-Chibi, an Arduinoid with open source 802.15.4 protocol stack and integrated radio. Akiba says:

The protocol stack uses approximately 3.6kB of flash and 270 bytes of RAM and makes it possible to use a bare radio for 802.15.4 communications rather than a module. There’s an accompanying board that I released which is Arduino-based with an integrated 802.15.4 radio. I also added a couple of options like a ruggedized enclosure and battery boost circuit. I designed the stack and the board to make it easy for people to set up wireless sensor networks or get started in embedded wireless communications.