I have a slightly different take on this.
You definitely need an ultra portable laptop with you all the time in school, while travelling etc for which a Thinkpad x201 12" laptop is perfect. In addition to that I am carrying my desktop with me. I added a Samsung p2570HD 25" LCD monitor/HDTV to it, which gives me a monitor as well as a full TV. I will be using it as my TV/DVD player/Music center and also for work related stuff like big excel sheets, heavy simulations or anything else that the 12" laptop is too small for. I kinda like sitting on a desk with a proper keyboard and good posture for long sittings.
Somehow, I prefer this combination over carrying a 14"-15" heavier laptop.
My preference is similar to yours. I can't even imagine myself using a laptop exclusively, but I can see how other people choose to do so. I'm a little concerned about the ease and convenience of transferring working documents between my laptop and desktop though. I guess that's what Dropbox is for...
Here's a link if anyone's curious about syncing files on multiple computers.http://www.dropbox.com/
GMAT Club Premium Membership - big benefits and savings