Free version available
You might want to pay attention to whether the license permits "free" commercial use, as well as hobbyist/educational use. It can be quite a moral quandary to be faced with the possibility of selling a few of your arduino-based gadget, only to need to spend thousands of dollars of licensing fees to stay compliant.
What do you think about Contiki? I know it's already been ported to mid-sized AVRs (Raven) to implement wireless IPv6 sensor networks; Networking seems to frequently be one of the factors pushing people to real OSes.