I have now sucessfully written a driver for Microchips MRF24J40 802.15.4 transceiver that work togheter with the C18 port of the uIP-stack. It's now possible to send ipv4 packets like UDP and TCP over 802.15.4! There are a lot of limitation and assumptions, but it works. I have added the driver to the uIP-stack, and there are a test project at SVN. Next step: 6LoWPAN.