Apple made Xcode available to everyone in exchange for a small payment on the Mac App Store, but now they have decided to change that and offer it completely free of charge, thus encouraging the development of apps.
To download Xcode, therefore, all you have to do is follow this link that takes you to the Mac App Store, where you can get hold of the Cupertino development suite.
Discharge is heavy, so give it time. Xcode 4.1 is the only version that Lion supports.