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Note: This solution uses Firefox, but it should work in any other browser that supports View > Full Screen.
Open Firefox to a new, blank tab (Navigate to about:blank to force a blank tab.). Press Shift+⌘+F, or go to View > Full Screen. Voila! Now, ⌘+Tab to the application you'd like to record. This works well for capturing the iPhone Simulator on a white background. If you'd like a different background color than white, then use Firebug, or just create a simple text file with the line <html style="background-color:#35ef8d"></html> (Change #35ef8d to any color you want.), save it as background.html, and open it with Firefox.