Where is the Xcode installer located? When I browse the Mac App Store, the buy button of the Xcode app is shown as "installed" so that I can't buy it. But I can't find it in my Applications folder either.

How can I locate the app? Or how can I change the "installed" button to "buy" button?
FYI, I had a similar problem but eventually noticed (via spotlight search for 'xcode') that rather than installing the actual app, it had downloaded an app in /Applications called, "Install XCode" which was 4.58 GB.

Running that does the actual install of XCode 4.