But it might also be possible by registering a tap by a mic on the side of the screen and connect that tap to a certain task.

You can add this to a standard macbook, whackbook pro

well i bet if you put some sort of vibration detector it would get a simpler effect because win you tap your acrylic it would send a vibration threw the acrylic and it would pick it up and if you put one on each side you could determent which side was taped and everything but that is just and idea i do not know if would work

on a multitouch display, you would rather do a little drag on the right spot than slapping something to switch desktops. but the idea to realize it on the mac with built-in-shock-detection-what-so-ever electronics is nice indeed..

This technique is used already. I have seen it in the AEC (http://www.aec.at) some years ago. It was developed by ESA and is now available as a commercial product.

http://www.sensitive-object.com/SO_ENG/M04/Produits.html
the Original:

http://www.esa.int/esaCP/SEM60JYEM4E_index_0.html#subhead3

tetratoon