Starting October 13th, World of Goo will be available to purchase on Steam, Direct2Drive, WiiWare, Greenhouse, and on 2D Boy's site as well. A boxed version can be pre-ordered from amazon.com if you're looking for a physical copy of the game.

The good news is that keys for pre-orders can be used to activate the game on your Steam account, although no demo for World of Goo should be expected until early next year.

where to get the goo (2D Boy)