However, since late 2009, Apple now lets you use another Mac's monitor, specifically the 27" iMac varieties, as a secondary display. With a cable and a keyboard press, an iMac can be re-used as a second monitor.
This feature is aptly called, Target Display Mode. Apple posts info on it here: http://support.apple.com/kb/HT3924
Plug in a Thunderbolt/MiniDisplayPort cable on a macbook and connect to an iMac.
Then press Command-F2 on the iMac. Voila. a Spanning display for Macbook Pros and Macbook Airs.
Now, Apple needs to implement on this on iOS devices. In an earlier blog post, I wrote about using an iPad 3 as a portable Cinema Display Monitor-to-Go using software.
With the new digital Lightning connector on the iPhone and 4th gen iPads, they need to add Mobile Target Display Mode Pronto!
Imagine it below as it is pictured below (using Air Display). 2048x1536 via Thunderbolt-to-Lightning would kill off any DisplayLink adapters/hacks.