My Macbook Air 2010 11in overheats when I close the lid and run an external monitor I often run my Macbook Air 2010 11 inch with an external monitor and keyboard, and I close the lid so that I'm using only the external monitor. But then the fan starts running on high in a few seconds, as if the laptop is not getting enough ventilation. Is this normal? 

I would think that the Air would be designed in a way to allow it to be ventilated adequately in this kind of use case.
Just because the fan starts running faster does not mean that the machine is overheated.

If you really want to, you can open the Air's lid after the external monitor is connected. The Air's screen will stay off but the machine should stay cooler and not have to run the fans at a high speed.

Apple even has a knowledge base article for "clamshell mode".