I've run an iMac as an external display for a Mac laptop before, but am wondering if my new setup can work like that. I have:

* 2011 Macbook Air with Thunderbolt
* 2009 iMac with Mini DisplayPort
* mini DisplayPort cable

Can I drive my iMac from my laptop? I've had no success so far, despite manually putting my iMac in Target Display Mode using cmd-F2. I can get the setup to the point where both screens turn blue (as expected) but then my iMac reverts to showing the iMac's desktop, not that of the laptop.