Comment 7 for bug 597070

This issue is affecting me too. I verified that this is the exact behavior I'm seeing with my iMac 11,1. This issue does not affect all model iMacs. Mine is a 27" late 2009 model.

Here is how to check the serial number to determine the model.

On the bottom of your iMac stand, you'll find a label with the serial number printed on it, or from the Apple menu, choose About This Mac. Configure To Order (CTO) models are designated with an asterisk (*). Check the last three characters on the serial number:
iMac (Mid 2010)
DAS, DNM* iMac (21.5-inch, Mid 2010)
DB7, DNN* iMac (21.5-inch, Mid 2010)
DB6, DNP* iMac (27-inch, Mid 2010)
DB5, DNR* iMac (27-inch, Mid 2010)
iMac (Late 2009)
5PC, B9U, CY8* iMac (21.5-inch, Late 2009)
5PK*, B9S* iMac (21.5-inch, Late 2009)
5PE, 5PJ, CYB* iMac (27-inch, Late 2009)
CYC*, 5PM*, 5RU* iMac (27-inch, Late 2009)