One of my initial concerns when I found out that AppleTVs were going to play YouTube videos, is that you wouldn't be able to easily search videos. As it turns out, searching is pretty simple. You use the Apple remote to select the letters and spaces of your search phrase. You can then navigate to the area on the right and select a video from the list.
Each time you enter a letter, the list on the right narrows down to match your search. This process is surprisingly fast. It shows the know-how of both Apple and Google. Once YouTube's full catalog is available through AppleTV, it will be an interesting comparison to see if navigation is still as speedy.