I found this on Makezine today. It's a work in progress, but the guy's building a sentry gun out of a Vulcan. Pretty neat.
http://mt.makezine.com/cgi-bin/mt/mt-search.cgi?blog_id=1&Template=tag_display&tag=@arduinonerf
http://mt.makezine.com/cgi-bin/mt/mt-search.cgi?blog_id=1&Template=tag_display&tag=@arduinonerf