Sorry for the bump, but I just finished Shantae: Risky's Revenge on DSiWare and it is absolutely a Metroidvania. The map is 2D but has multiple layers, sorta like an advanced version of Goonies 2 NES. As you play through the game, you find new transformation abilities that allow you to traverse previously inaccessible parts of the map. You get the ability to climb walls and jump high like Grant in CV3, break through stone barriers like in CotM, swim like AoS and DoS, etc. In fact, the basic attack mechanic is a hair whip, so the gameplay is a little CV-esque. It's a very polished game. I picked it up because I was blown away by Wayforward in Contra 4, and I was looking for their other stuff. I highly recommend Shantae to any Metroidvania fans. It's a beatifully animated 2D piece of artwork.