This is exactly why the 3G model is the iPad to buy, unless you're positive it's never leaving your Wi-Fi-covered house. You can buy data and cancel at any time, right from the iPad.
Look at it this way: That $100 up front for the 3G model? It's insurance. Because if you need 3G and don't have it, you're just screwed. But if you have it and never use data, it's no bigs, 'cause you're not on a contract. [Apple via MacRumors]