The recent update to Xcode 4.3 marks a change in how Apple sets up developer systems. Until now all development-specific stuff (libraries, headers, Xcode itself, …) resided in /Developer. With 4.3 that directory is obsolete. The new directory is:
So if you use command line build tools like xcodebuild and see errors like this:
Error: No developer directory found at /Developer. Run /usr/bin/xcode-select to update the developer directory path.
… you might need to tell the system about the new path:
sudo /usr/bin/xcode-select -switch /Applications/Xcode.app/Contents/Developer