I hit a small road bump while re-installing Xcode 4 after a clean install of Mac OSX Lion:
In order to continue the installation please close the following application: iTunes
I quit the application but the alert box did not go away. To the terminal…
Dauphin:~ james$ ps x | grep iTunes 237 ?? S 0:00.29 /Applications/iTunes.app/Contents/MacOS/iTunesHelper.app/Contents/MacOS/iTunesHelper -psn_0_86037 13970 s000 S+ 0:00.00 grep iTunes
Aha! The iTunes helper application was still running preventing the installation from continuing.
Dapuhin:~ james$ kill 237
Problem solved.
UPDATE:
Apple has released a delta update (4.1.1) to Xcode to fix this problem:
How nice of them.