MacOS – Copying Xcode installation from one Mac to another

I know it's a bit confusing, but you didn't actually install Xcode 4. What you did was installing the Xcode 4 installer. So what you got to do is:

1. Open the Install Xcode-app located in your launchpad.
2. Make sure iTunes is closed, this is necessary, and the installer won't check for this.
3. Finalize the setup (well, Lion will do this for you, just press the buttons).
4. Optionally, restart your Mac.
5. Optionally, remove the Install Xcode-app.

It'll probably prompt you that it's moving your existing Developer-folder to Developer Old so it can install Xcode 4.1 to /Developer/.

Yes

You can simply copy the application to your Applications folder, and then login to the Mac Application Store with the same username and password. This will validate the application and allow you to run it without needing to download it again.

The only time this will become a legal issue is if you are copying it to someone else's computer, however, they will not get updates for the application unless they log in as you, or otherwise, unless they also have a purchased copy of the application.

You can also read my answer here for a more detailed explanation of Application installs on the Mac and the Mac App Store.