According to Michael Wilford, CEO of Twisted Pixel, Splosion Man will be the first game that allows you to unlock Avatar Accessories by playing through the game. The game comes out next week for 800MSP but will have a title update when the Avatar Marketplace update ships that will enable you to unlock the items.
As to what these items are going to be, you can guarantee I will post it here as soon as I know. The game will also feature an unlockable premium theme just as the maw did along with some gamerpics.
Source - majornelson.com/archive/2009/0...