Lion has this feature to switch to the external display when you close the lid of your Mac laptop. It seems like a welcomed feature at first. I do a lot of screen sharing over iChat/Messages Beta, and had to switch from dual-desktop to single-screen for screen sharing to work nicely. In Snow Leopard I had to close the lid, wait for Mac to go to sleep, and then wake it up with my keyboard or magic trackpad. However, there’s a huge problem with the new clamshell setting in Lion – the machine gets very hot when used in clamshell mode, especially when you do development on it such as hacking in Xcode, with the iOS app running in the simulator, the app itself talks to a local Rails app served up with Pow, and the Rails app also talks to MongoDB or MySQL running on the same machine, Redis, and a whole lot of other standard techs we use to get things done.
Today I finally got fed up and switched back to the way Snow Leopard worked, there really isn’t a documented way to do this, but you can get the old behavior back by changing the nvram boot arguments:
sudo nvram boot-args="iog=0x0"
Bounce the Mac after that and you’ll get the behavior back.