I just upgraded my MacOSX from Snow Leopard to Mountain Lion. The previous version of my XCode is 3, and after the upgrade of MacOSX, I installed XCode 4 as well.
However, when I try to use the new XCode to develop a helloworld-like iphone app, I got this problem. The interface builder process takes 100% cpu and never respond. So for now I cannot do any development job on this computer.
I rebooted the mac and the problem is still there.
Does anyone have ideas why I cannot launch Interface Builder successfully?
Do you have enough RAM to run 10.8 and Xcode together? I would suggest killing background programs or upgrading your Mac, because Xcode never uses that kind of resources.