I am also evaluating RTOSs available for the AT91SAM7X. freeRTOS seems to work, but I'm not a fan of documentation via wiki.
Check out uTasker,
http://www.uTasker.com. I'm still new to it, but I really like the documentation. Also the email support has been absolutely amazing! I wouldn't be surprised if we end up using this in a commercial application. The price is low, $765USD for a full license? nice!
Mark