I just had a problem with Xcode 4.0.2 crashing on startup after installation, and thought I'd share the solution. I had just upgraded (in-place) from Xcode 4.0.1. I installed Xcode 4.0.1 in /Developer-4.0, not the default /Developer.

I looked at system logs in Console, they suggested Xcode crashed becaue dyld couldn't find (and thus load) CoreSymbolication.framework.

I figured the ran the uninstall script in Terminal.

sudo /Developer-4.0/Library/uninstall-devtools

After that completed successfully, I trashed the /Developer-4.0 folder (there were some useless leftovers), and re-installed Xcode 4.0.2. It worked!