Apple's free Remote app allows you to control the music on your computer or Apple TV from your iPhone or iPod touch, but with the new 1.3 update, it is now able to control more than just your music.
With the 1.3 update, the app now supports the ability to control your Apple TV with "simple finger gestures." That's pretty much the entire description of the update, but it seems pretty self explanatory. The app also now requires iPhone OS 3.0, so if you don't have it or don't intend to upgrade to it, don't download the update.
If you have already downloaded the app, just check for the update from the device itself as it seems it hasn't propagated to all of the servers quite yet. If you haven't downloaded it yet, just download the current version and check for updates the same way.
[Thanks to mmcnulty7 for the tip!]