I recently bought Xcode 4 on the Mac AppStore and thereby thought I’d upgrade. Nope. Xcode 3 is moved to ‘/Developer-old’, but kept. No big dead actually, except when your OS Disk is only 60Gb. The new Xcode 4 uses almost 10Gb plust 5Gb for Xcode 3. So if you don’t need Xcode 3 anymore, just run:
sudo /Developer-old/Library/uninstall-devtools --mode=all
This removes all Xcode3 files, freeing up about 5Gb of space.