ACD 27" not outputting beyond 1920x1200, why?

Greetings,

I just purchased a new Mac Mini Server 2011 and a used (but still on AppleCare) LED Apple Cinema Display 27". They are connected via the Mini DisplayPort cable running off the display into the Thunderbolt port on the Mini, but 1920x1200 is the as max resolution I can set the screen to. I do not have the option to select 2640x1440. What am I doing wrong?

Cheers, Dan