Just came across this issue myself, the problem was that I had a Entitlements.plist file in the project as part of an ad hoc distribution, and its get-task-allow (ie. ‘can be debugged’) property was set to NO – setting this to YES fixed the issue and allowed the app to run from Xcode4 on the device in development.
Naturally need to set it back to NO for ad hoc distributions, but just thought I’d mention it in case anyone else comes across the same problem.