Hahahaha that probably makes some people jealous, smaller than a mac mini.
I keep my macbok open. Firstly, I don't have a USB keyboard. And when I will, I'll probably keep open next to my 22" display. Hey, extra space, why not !?
I usualy keep my main work on the 22" and then swoop things that don't have to do with it on the macbook or if I don't have the space for it on the 22". Using audio software, watching a movie or playing games isn't fun when you shrink the window to put other stuff. It also removes the wrist job of opening exposé, I can see pretty much all my windows all the time. Mind you the macbook does get cluttered, having iTunes, firefox and adium open all at once, I actually have to flip windows or use the mini player. It can both be a plus as I can see all my info at once but I guess that since I don't really need it, it could cut in my productivity at some point, great time waster.
Once I do get a keyboard, I'll probably center the big display and keep the macbook on the side. That said, I like having both the touchpad and the mouse. Being able to do pretty much all my surfing on the pad (tab flipping, going back and forth) and a couple of time savers with multiclutch.