Type the phrase "do a barrel roll" (without quotes) into the search box, and if you are using Chrome, you, um, get what you asked for. Unfortunately is does not work on Firefox, IE or Safari.
Update: Just to confirm that it does now work in Firefox (didn't at time of writing) and it also definitely works in Safari on iPhone. Apparently it works in Safari 5.1 (OSX 10.7) but we have not been able to recreate that.
Can you find any others? We have tried "do a backflip" and "do a handstand" to no avail.
Update: This Google Easter Egg is actually a reference to the Nintendo game, "Starfox", in which your Rabbit wingman Peppy tells you to "do a Barrel Roll" to evade enemy missiles. Perhaps there are more computer game related Easter eggs hidden in Google's algorithm?