There are probably tons of users out there who run more than one monitor, going two monitors these days isn’t completely unheard of. Three screens? Maybe pushing it a little already. You should know that if the reports are right, Mac OS X Lion does not always take advantage of a multi-screen set up.
When you toggle and app to “full screen” mode while running two monitors side by side, the app will fill up one monitor as expected, but instead of leaving the second monitor alone with whatever icons and folders and wallpapers you would come to expect to see in the background, OS X Lion displays the famous linen cloth wallpaper and basically disables the second monitor until you go back to “windowed” mode. Why? Probably for aesthetics reasons.
Let’s just hope that this is only because OS X Lion is still in beta because if not there will probably be plenty of Apple users who have gone out and spent $1,000 on the 27-inch Apple LED Cinema Display only to find themselves staring at 2560 x 1440 resolution linen cloth wallpaper.
It also seems like placing an app in fullscreen mode onto your secondary monitor will immediately send the app over to the primary monitor, which might not be what the user wants…