(OS X, not iOS): "Choose a destination with a supported architecture in order to run on this system"
Fixed the copied .plist in the Build settings, removed spaces from the new product name. Can't find any difference with the original target's settings, still this dreaded error. Any ideas?
With Xcode 4.5.2. (4G2008a) this issue has gone away, apparently. I can now make a duplicate of the original target, fix the product name, and get a properly running application.