This is great -- developer Wannes Vermeulen has hacked a toy remote control car with an Arduino board and hooked it up to an iPad to drive it around. You can see video of the project in action over on Vimeo right now. It's not anything too special these days, as you can buy lots of RC vehicles that are controlled with an iPad or an iPhone app, but as a DIY project, it's pretty amazing.
There's actually an Android smartphone on the car itself to provide camera information and wifi access, and I believe, according to the description, that it also connects to a laptop, which then sends information to the iPad. So it's a pretty complicated setup, especially given that there are vehicles which will do this out of the box. But making it from all of these pieces is really cool no matter what.