I don't know if this is true; we've only got two Air's and I think only one is in use:
Want to Speed Up Your MacBook Air?
Try a BookArc. Really.
BookArc for MacBook Air is more than a beautifully designed stand for your Mac. If you work with an external monitor, BookArc can actually improve the performance of MacBook Air. How? When you work with two monitors, your Mac’s video memory is split between the two, but when you work with your MacBook closed, your Mac only has to power the external display. You will notice an increase in speed, especially when working in Photoshop, Aperture, iPhoto and other graphics-intensive apps. So can a notebook stand really speed up your MacBook? If it’s a BookArc, the answer is yes
http://twelvesouth.com/products/BookArc_for_Air/The same company sells a little shelf to stick on the back of the stand of your iMAC: