This bar (Kingston KVR1066D3S7 / 2G) is running properly in the new iMac 27 "Late 2009, c2d 3,06GHz. After installation (this is the only thing that Apple allowed), latches (2x2GB) were recognized equal. Performance difference: (load iPhoto, iTunes, Photoshop) in complex applications, it has changed noticeably for me - positive. The iMac has become much faster.
I can only recommend everyone to treat themselves to an upgrade to 8 GB RAM