Since the 4.3 version of Xcode on Lion, it no longer resides in the Developer folder in the root of your hard drive, but as an app in the Applications folder. It also no longer installs the Command Line Tools by default. This in turn breaks many of the dependencies of MacPorts. The first step to fix this is open Xcode and go to Preferences in the Menu Bar under Xcode. Once that is open go to the Downloads section and click Install next to Command Line Tools.
After that is done, open the Terminal and type:
sudo xcode-select -switch /Applications/Xcode.app
Press Enter, type your password and now MacPorts and Xcode Tools should now work together like normal.