I've experienced the exact same thing running a 24" LCD, firefox, and Entourage. Hardly an uncommon or extreme setup, but it's enough to throw the system off kilter. And when kernel_task goes into "cool this thing off NOW" mode, the system is unusable for 4-5 minutes.

The MBA is a beautiful monster.

I think there's a disconnect here. Yes, it comes with Xcode. Yes, it's advertised as being able to run a large external display. But the question is: "can it do both of these things (in the case of Xcode, most likely building some complex project) simultaneously, and persist in so doing for an extended period of time?"

It appears the answer to that question is "no", and frankly I don't find that particularly surprising. I also don't think it really contradicts the marketing materials. Consider a different example: the MacBook Air comes with a wide variety of applications preinstalled, and this is an advertised feature of the laptop. But if I attempt to launch and use them all simultaneously, the system will become unusable very quickly as it runs out of RAM and has to start swapping. Does that mean Apple is falsely advertising its capabilities by touting the applications which come preinstalled?

or a £2k macbook pro.

The fact that the air can't seem to handle a second framebuffer and edit text at the same time is a little ridiculous.