Along the lines of Semacode, another "use your phonecam as a meatspace remote control" project -- Spotcode. Developer Anil Madhavapeddy says:
I've been working on some software that lets you use your existing camera phone as a virtual mouse by locking onto tags and physically rotating it around and so on. It's most easily explained by checking out the videos. In particular, the volume control one (MPEG) is fun.
Link (Also spotted on Warren Ellis' blog)