Important Update
The strange bug is fixed (at least for me) in Xcode 4.2!
I\'ve a serious problem with Xcode 4.1 Build 4B110 under Mac
I had the problem of instruments locking up and the above cmd-tab trick didn't work until I did it such that the dialog "Instruments is trying to access your computer" appeared asking for my password. After I entered the password then I could get to instruments. I suspect the problem is the password dialog is hidden in the Xcode windows. In this case Instruments is frozen because it has the modal dialog open.
So:
This may be a one time thing... now that I gave Instruments access it may not need the password again.