It's no secret that Paradox is looking to get Magicka on "all consoles". When reached for comment about the ESRB rating, Paradox producer Shams Jorjani told Joystiq the the company is "covering [its] bases." He added that the game was built using XNA so it will run on Xbox 360. He concluded, "We are doing everything we can to get this game where it belongs: In front of your couch."
It's worth noting that Paradox originally intended to release its class-based shooter, Lead & Gold, in a more multiplatform manner. It was eventually released for PC and PSN, and the XBLA version never came to light. Here's hoping Paradox has ironed out the multiplatform kinks this time around.