After installing Xcode 4.0.2, I get this error when I try to launch it by double-clicking on an existing project:
Dyld Error Message: Library not loaded: @rpath/DevToolsFoundation.framework/Versions/A/DevToolsFoundation Referenced from: /Developer/Developer/Applications/Xcode.app/Contents/MacOS/Xcode Reason: image not found
Launching Xcode via the dock and opening the project from inside Xcode works fine. Also note that doing a repair install of Xcode did not resolve the issue. I haven't tried completely uninstalling Xcode and installing from scratch, but I would prefer to avoid that as I have my IDE nicely customized the way I like it.
Update: I just tried a complete uninstall of Xcode via sudo Developer/Library/uninstall-devtools --mode=all then rebooted and reinstalled. Still having the same issue.
Before you reinstall, make sure your project file is set to open with the expected copy of Xcode 4. Select it in Finder, press Cmd-I, and make sure the Open With... popup is set to the right copy of Xcode 4.