How to transfer Xcode installations between Macs?

When you use Xcode 4 everything you really need, is in the Xcode.app bundle in your Applications folder. You can just copy this to your new computer.
If you want you can also copy ~/Library/Developer/Shared/Xcode where plug-ins may reside.
An other interesting folder is ~/Library/Developer. There are amongst others Docsests, iOS device logs, etc. stored. You may take a look yourself in there, to see if you need to copy this.