I had the same problem a few days ago after the 10.7.3 update. if you use the 10.7 SDK (the target could be 10.6 if you want to access Snow Leopard users)
The problem is that the update was originally 11D50, which was quietly updated to 11D50b. I think, for some reason, the Apple server or Xcode thinks that you are using the beta version of Mac OS X (10D50b), because "b" at the end may indicate the beta version.
The workaround was to change the system version in /System/Library/CoreServices/SystemVersion.plist from 11D50b to 11D50
I needed to use sudo to modify the file
sudo /Applications/TextEdit.app/Contents/MacOS/TextEdit /System/Library/CoreServices/SystemVersion.plist
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