iPhone: Building Full and Lite Versions from Same Source Tree

Here are some articles that explain how it can be done:

gamesfromwithin.com/from-full-to-lite-in-under-an-hour
adeem.me/blog/2009/04/18/tutorial-same-xcode-project-create-multiple-products-for-iphone/
stackoverflow.com/questions/549462/how-do-i-manage-building-a-lite-vs-paid-version-of-an-iphone-app