How can I mirror the display from an iOS device to an Apple TV? Is there a way to mirror the display of an iPod Touch or iPad to the Apple TV using AirPlay?

I know I can do it for Applications that support it, but for presentations it's often useful to just mirror the whole display, e.g. when wanting to show something in Safari.
If you have an iPad 2 and a iOS developer's account (or patience until this fall) you can try the hot new iOS 5 feature:

AirPlay Mirroring for iPad 2