Can my Thunderbolt Macbook Air drive my Mini DisplayPort iMac as an external display?

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.

According to this source, this should work. This other source confirms it. These are the troubleshooting steps I would try:

• Run all software updates on both the iMac and MacBook Air. A lot of times there are firmware updates that address issues like this.

• Open System Preferences > Displays and click Detect Displays.

• Reset the PRAM on both the iMac and the MacBook Air by restarting the computer and holding ⌘+⌥+P+R on boot but until you hear the POST chime twice. Then you can let go and boot normally. Be sure to do this on both machines.

Try going through these steps and let me know if you're able to resolve the issue.