We've gushed at length many times before about Adult Swim's Robot Unicorn Attack [$2.99] for the iPhone between our initial preview, release announcement, its subsequent updates such as when it gained OpenFeint and even the recent announcement that Robot Unicorn Attack Heavy Metal was in the works. If this is the first time you've heard of it, you really need to just stop whatever it is you're doing and get to a browser with Flash to play the free online flash version.
The gameplay of the recently released Robot Unicorn Attack HD [99¢] is identical to that of both the iPhone and flash versions of the game, except it runs at 1024x768 and the graphics are noticeably improved. The robot unicorn is much better looking, along with everything else. The terrain looks a little rough in screenshots, but it isn't apparent at all when you're actually playing the game.
If you own an iPad, you need to have Robot Unicorn Attack HD on it-- especially at its launch price of 99¢.