Apple 30" display and iMac 27"
Apple's tech spec page for the Late 2011 iMac and the new thin iMac say that they both support both the built in monitor and an external monitor at up to 2560X1600. It seems like the Mini display port to Dual Link DVI adapter should work. This conversation on Apple's support site concurs.

Double check all of your connections, try rebooting with the monitor plugged in and if you still can't get it to work try taking to to a genius bar if you have an Apple retail story nearby.