I am a big fan of the Amped series but with no multiplayer support, why would anyone want it? The only 2 player action is a sledding mini-game. You can't even board split screen with a buddy. I wish they spent less time on the cartoon cut sceens and voice overs and more time implementing multiplayer.
From an interview with Amped 3 Senior Producer Matthew Seymour:
What about offline multiplayer? Have you incorporated any game modes for those of us that like to kick it old school?
Matthew Seymour: We’ve incorporated the new Sled Slaughter mode, where two players will each control one of two sleds that are tied together, and people have really enjoyed this mode so far. This is what you’ll be playing side-by-side with some buddies.
We had grandiose plans for other multiplayer modes, both online and offline, but there simply wasn’t enough time. That being said, there is so much to Amped 3 that I think passing the controller around and trying to complete challenges with several people will be just as fun.
http://interviews.teamxbox.com/xbox/1347/Amped-3-Interview/p1/