I spoke to someone recently who quoted David Rock as suggesting that the pre-frontal cortex (the part of the brain responsible for thinking as opposed to memory) is a little like a stage in a theatre, with the actors on the stage being the things you're currently thinking about. Too many actors clouds your thinking, which is why when you get to work in the morning it's a useful trick to get some larger piece of work out of the way rather than diving into email! The audience in the theatre are your memory; to recall something, you bring an audience member up onto the stage.
The computer "desktop" works in a similar way. Have 20 windows open, and you're likely not only to spend time hunting for the right window or getting distracted by other windows, but also (in my opinion) you will worry in your subconscious about background windows you're not using.
But large computer screens are very useful in showing more of the window you're working with (so that you have more context - or more of a rounded character for your "actor", and have to remember less) and of course showing one or two windows side by side, related to whatever you're working on, which has the same effect.
Having tried out a 10 inch laptop for various pieces of work (not just programming but document writing, emailing and so on) I can safely say I'm not interested in a tablet PC (of any size, let along a 10 inch screen half obscured by an on-screen keyboard).
Many of my acquaintances, however, tell me how their tablet PC has changed their life...
So what do you think?