If you purchased the PC version of Super Amazing Wagon Adventure before the Steam launch and want a Steam key for the game, this page tells you how to get one. How to get your Steam key depends on where you bought the game.
Desura: Visit www.desura.com/collection while logged into your Desura account. Find Super Amazing Wagon Adventure and click "Keys" and there should be a button to get a Steam key.
Indie Royale: Visit the link you got to download the game when you bought the bundle. If you lost that email, go to www.indieroyale.com and click "Lost Key?" at the bottom of the page.
GamersGate: Log in to GamersGate, click My Shelf and the game's name. You should see a link to get a Steam key on the right hand side under the Download button.
Xbox Live Indie Games: I'm not currently giving Steam keys to people that purchased the Xbox version. Sorry! For various reasons the PC and Xbox versions of the game are priced differently, so giving away Steam keys to Xbox owners wouldn't be fair to people buying the PC version of the game. However, aside from some PC specific options, the PC and Xbox versions are the same, so there's no reason to buy the game again if you have the Xbox version.
I think that covers it! If you have problems getting your Steam key, send an email to sparsevector at sprasevector dot com.