I installed Xcode 4 and now cannot run one of my iPhone projects.
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I ran into the same issue. Go to Product -> Edit Scheme and select the Run scheme on the lefthand side. Go to the info tab, and next to launch there will be two options (neither of them selected): 'Automatically' and 'Wait for XXXX.app to Launch'. Select the Automatically one and it should clear up your issue.
I had actually built and used my projects using xcode 4 for a few days with no issues, and then this popped up randomly. Hope the this helps.