I agree on something cheaper that the current WSN motes around at the moment, this is one of the reasons I've been working on the widget, something I can build and afford to play around with on a larger scale around the house.
I did a preliminary design for a 2.4ghz 802.15.4 widget board some time ago based on microchips FCC approved MRF24J40MA module, this has an onboard AES function, as part of the 802.15.4 specs so it can encrypt the packets payload, or you can use it as a function where you can encrypt data, just takes up SPI cycles and returns the encrypted data.
I added a few of these PCBs to my initial widget run but have not yet assembled them :( time time sigh!
Another option is the new RFM22/23 series that are a bit smarter than the RFM12 series. Not 802.15.4 compliant, and no encryption, but has a propper digital RSSI and alot of other nice features. BTW you can hack a RSSI signal from the RFM12 SMD version by connecting an AD port to a resistor onboard the RFM12 (http://blog.strobotics.com.au/2008/06/17/rfm12-tutorial-part2/) there was a couple of comments on this posting regarding their success in reading this value. (I've not yet tried it)
Many thanks Jean-claude, my c skills are pretty rust but are coming back to me lol. I'm actually on leave at the moment and have had some time to put into coding the driver. I must admit porting the widget to contiki has certianly been a learning experience for me, mainly trying to work out how contiki fits together.
Anyway I've just about finished the driver and hopefully can test in the next couple of days. Once that's done then into the upper layers, which will be hardware in-dependant so can be developed and tested without the need for the hardware until final testing.
If anyone wants to put their heads together in finalising a protocol please let me know.