This is actually several kinds of neat: a Magic Mouse accessing an iPhone, via Bluetooth, courtesy of the BTStack project. (No, we didn't know what BTStack was, either; according to the project's home page, it "provides a portable Bluetooth Stack [and supports] devices for which the OS either does not provide a Bluetooth Stack or the available stack is severely limited [e.g., on the iPhone].")
But it's not just the Mouse that's intriguing in this demo. Notice the red glowing thingie on the desktop? That's a Celluon CL800BT virtual keyboard, which can project onto any flat surface and respond to your touch.
While you may question the actual, y'know, need for mouse support on your iPhone, the BTStack functionality alone is going to open up lots of connectivity possibilities. (Plus, if you've ever struggled with an old Palm Pilot/expansion keyboard combo, just the idea of a virtual keyboard like the Celluon is way cool.) And if you've got a jailbroken iPhone, you can already link to a Bluetooth keyboard — here's how.