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Apple’s OS X Lion will allow you to customize backgrounds for each desktop that's open in Mission Control. This helps you to recognize specific desktops at-a-glance. Here's how.
Log into your Mac and open System Preferences.
Click Desktop & Screen Saver and then select the Lion under Desktop Pictures.
On your track pad swipe three fingers up to open Mission Control.
Open a second desktop by clicking the plus pop-up that appears on the upper right side of the Mission Control screen. This will add another virtual desktop.
Click Desktop 2 to open it.
Open System Preferences again and click Desktop & Screen Saver. Now select the Elephant under Desktop Pictures.
On your Track pad swipe three fingers up to open Mission Control.
Each virtual desktop now has its own unique background. Cool!