From the MAKE Forums:
If you’ve been looking for a way to control your robotic project over a WiFi, then you might want to take a look at the Remote Control Over IP (RCOIP) project by Mike McCauley. He uses a lot of complicated words, but it’s basically a communication protocol to do RC-style control over an Internet connection.
Of course, you can’t connect directly to a traditional RC device using your laptop, so you will need to substitute your radio receiver for an Arduino with a WiFi shield, and put the RCKit library on it. For the controller end, you could use another Arduino or a computer. He’s also got a (nonfree) iPhone control app, which allows you to control your creation using a simulated touchscreen RC controller.
There are probably a bunch of different protocols that people use for this, such as Open Sound Controller– do you have a favorite?