If you are unlucky enough to have installed XCode from the Developer Network, instead of the new Mac App Store, you may be seeing this message:
You have updates available for other accounts.
This may be followed by either an actual account email address, or by a generic message saying to “sign into the account you used to purchase it.”
Either way, it’s a problem. You can’t install or update XCode until you delete the “Install XCode.dmg” file which is usually found in your /Applications directory, but which was in my Downloads folder, because I copied the DMG file from someone else with a local connection, rather than download it from the Developer site again.
Bit of a stupid problem, but luckily it has a simple answer.