In the supporting document to Apple on the issue, the notes of the society that users must also leave the iTunesHelper process, which can be found in and out of the Activity Monitor utility. Once iTunesHelper has been quit, the installation continues normally.
Apple today addressed the behavior creates confusion with the release of Xcode 4.1.1 by the Mac App Store, a small delta update which removes the alert facility which has caused problems for users. Users who have already installed Xcode 4.1 are not required to install the updated version. What's new in Version 4.1.1
This is an update from the delta to install a Xcode.app which fixes the "Installation Alert" stop iTunes. You do not need to rerun the Setup program if you have Xcode 4.1 for Lion installed successfully, as the included tools are unchanged in this update.4.1.1 Xcode is a free download in the Mac App Store, but requires the OS X lion.