Nothing. With Lion you cannot clone your startup volume when it's active. Do this:
Clone Lion using Restore Option of Disk Utility
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
- Select Disk Utility from the main menu then press the Continue button.
- Select the destination volume from the left side list.
- Click on the Restore tab in the DU main window.
- Check the box labeled Erase destination.
- Select the destination volume from the left side list and drag it to the Destination entry field.
- Select the source volume from the left side list and drag it to the Source entry field.
- Double-check you got it right, then click on the Restore button.
Destination means the external backup drive. Source means the internal startup drive.
The only thing I noticed that was not exactly as Kappy said is that the arrow button on the Recovery HD was pointing upward, not downward. Not a big deal but I thought I'd chime in just in case someone else sees the same thing. I just ran these steps to clone the HD from a late 2011 Macbook Pro 15, Model ID 8,2 onto a new Hitachi (HGST Travelstar 7K750 7200RPM). One other thing to note is when you quit out of Disk Utility when the clone is finished, you can set the new external drive as the boot drive to test that everything worked. This gives a pretty good idea that it will work if/when you swap the drives.