You may be holding your sparkling new copy of Batman: Arkham City right now, but thinking to yourself, "Gee, I wish Batman looked more like a short-lived 90s spin-off cartoon." Well sir, you are strange; but also, you are one energy drink away from having your wish granted. The Batman Beyond skin can be gained for a single bottle of NOS.
The NOS site (via Joystiq) lets you pop a cap and enter a code for rewards, one of which is Batman Beyond. The skin can be used once you've completed the main story and in the Challenge Maps. So if you want to imagine your New Game Plus playthrough involves a time-traveling, barrel-chested Terry McGinnis from the future, done and done.
It wasn't exactly clear how to get this skin before, since Rocksteady's complicated skin scheme didn't list a US release. Like the myriad other skins, this one is being offered early through NOS, but that won't be your only chance. All of the skins are timed exclusives, so Rocksteady is sure to bundle them together sometime down the line.