This is probably the coolest bit of iOS connectivity I've seen so far. KL Dartboard is an iPad app (for US$3.99) that serves as a virtual board for darts thrown by an app called KL Darts ... for the iPhone. So, the idea is that you buy the iPad app and set it up on the wall, and then you download the free iPhone app and connect the two using Bluetooth. Once the setup is complete, the position that you hold and shake the iPhone in determines where the darts land on the iPad's board.
What a weird concept. Unfortunately, it's probably a better idea than it actually is a game; the iPhone app requires OS 4, you've got to have both devices, and the reviews on the iPad app say it doesn't really work that well. It's impossible for the iPad to know "where" the iPhone is, so you don't even need to face the screen to throw the darts correctly.
Still, I think we'll see more cross-device interaction like this; Chopper 2 is a game that is due out soon, and it allows you to use your iPhone as a controller for the iPad version. I think there are plenty of possibilities like that for developers to play around with.