internal monitor not shutting off when lid closed + external monitor connected

So, like the title says, when I connect my Macbook Air to my fairly new Dell U2212HM monitor through Display Port (mini to regular cable), and I close the lid, the integrated monitor does not shut itself off anymore. I'm pretty sure it used to, but I'm not sure how long I've had this issue. Can't be much longer than a month or so.
I've tried having the Macbook opened and then connecting the cable + turning on the monitor, I've tried connecting while lid was already closed (in which case it actually doesn't even respond most of the time), and I've tried connecting and then rebooting. The last one seems to solve the issue sometimes, but not all the time.
I've also tried using a different user account btw, no influence there.
What could be causing this problem? I'm running an up to date os x lion on the newest version of the Macbook Air 13".

 

Yes it goes to sleep when there is no monitor connected.
When I use detect screens when the external display is connected and working, it blinks for a second, but keeps showing 2 active screens even though the lid is closed.

 

I finally found something referring to it.
It's basically this issue, and it's good to see I'm not the only one experiencing it: https://discussions.apple.com/thread/3435078?start=0&tstart=0
Unfortunately it seems there is no solution yet, or at least no one has a solution in that thread.