Trying to update Xcode 4.0.1 from within the Mac AppStore application, I was greeted with this error message:
You have updates available for other accounts Sign in to (null) to update applications for that account.
There exist numerous forum entries on Apple's discussion board on that subject and the recommended solution is to trash the "Install Xcode" application and then to re-launch the AppStore, selecting to re-install Xcode.
I trashed "Install Xcode" but AppStore produced the same message. Relaunched one additional time and it allowed me to update (note: not install). However, when it was finished, there was no new "Install Xcode" in the Applications folder!
It turns out, the AppStore had located and updated a different copy of the software on a separate drive where I had made a backup, even though I had renamed it to "Install Xcode-4.0.1". Its info window revealed that it had been modified and that its version number now was 4.0.2.
Installation from that modified backup worked correctly.
The old backup could be restored from the version that I had put into the trash.