Since it works over wifi and not bluetooth, the Remote app on the iPhone will control iTunes on the computer in my office even if I'm in the lab, around 300 feet away. (Possibly useful, I guess, as part of an elaborate prank.)

(BTW, the Remote app has my vote for the most useful development for the iPhone. 3G's irrelevant to me given the fact that I can barely get Edge coverage at my house; the nearest AT&T 3G is two hundred miles away.)