Apple's Remote: turns your iPhone into a WiFi remote control
Update: Looks like Apple has updated Apple TV to 2.1 with support for the new remote app, along with support for MobileMe. Get it via your Apple TV's Update Software feature.
Update 2: You can control playback of iTunes videos on your PC or Mac well -- a sceenshot of the video appears on the iPhone.
Gallery: Apple TV 2.1 Update
Read [warning: iTunes URL]