I had toyed with the idea of buying a Macbook Air and doing this. My issue is that my current laptop is 11.6'' and while I'm fine with that size on a cheap portable laptop, I feel like if I bought an Apple computer, I'd want something larger since I'd already be in a grand anyway. I think I'll end up settling on a Mac Mini and using my chromebook to remote into my Mac.
At any rate, I do dock my current laptop (11.6'' Samsung Chromebook) into a 24'' 1080p LED Samsung monitor and the results are pretty fantastic. The laptop's built-in display kind of sucks (bad viewing angles, bad colors) but it is adequate for mobile work. The external monitor as amazing for when I'm at home and need a better display because I'm watching movies or messing with photos or w/e
That is why I bought an 11 inch MBA. I have a 21 inch 1080p monitor with keyboard and mouse on my desk where I usually use my !machine. When I have to move around a lot nothing beats my. Ultra portable MBA.