@novice, vixr: Zigbee compatibility comes on two levels. On one hand you have the radio that implements the physical and MAC layers, and a network layer that performs routing, among other things. On the other hand you have modules such as xbee that allow you to talk over zigbee from an MCU, usually via serial. These modules have their own MCU that implements all the zigbee stuff in software.
If you save money by eliminating that MCU and doing everything on the AVR you're going to have to run a network stack on the AVR along with your application, so the second case of zigbee compatibility obviously does not apply. The first case is a matter of implementing a standard. The biggest problem is that if you run both the network layer and applications on one chip you're going to need something that enables multiple threads of control (and no, interrupts aren't going to cut it). Several such systems have been proposed using 'real' multitasking/threading (Mantis, freertos), protothreads (Contiki), and event-based programming (TinyOS). Personally I prefer Contiki although my goal is to just do everything in Java and use green threads.
So 'Zigbee compatibility' requires either a second MCU (which is what xbee modules have) or a more advanced 'os' or runtime or library or whatever that lets you do multithreading in a sane way. If you want complex network features you're going to need something other than the Arduino language. For robotics/wsn I'd suggest you take a look at Contiki (http://www.sics.se/contiki/about-contiki.html
), which has a wireless stack called RIME that's already running on the AVR Raven boards which use about the same setup as what we've been talking about (avr+atmel 2.4ghz rf), so should be trivial to port. It's IPV6 baby :3
For just simple point-to-point communication in the absence of another 2.4ghz network on the same channel you can just send raw packets over the radio. Control your Philips ambilight and such
It's perfectly fine for applications such as controlling your coffeemaker wirelessly. You only need Zigbee or something like that for large scale, multi-hop networks, or for integration with existing Zigbee stuff.
As for open sourcing I already open source my efforts under the LGPL license, my JVM can be found at http://darjeeling.sourceforge.net/