Problem getting 2560X1440 resolution on second monitor

I have an iMay 27 - inch late 2009 model.

The specifications state:

• Support for extended desktop and video mirroring modes
• Simultaneously supports full native resolution on the built-in display and up to a 30-inch display (2560 by 1600 pixels) on an external display

I have an external display. It is an Asus PB278Q. Wide Screen 27.0", According to the specifiations its resolution is as follows:

True Resolution : 2560x1440 (HDMI/DisplayPort/Dual-link DVI) 1920x1080 (D-sub)

The small print says "While the PB278Q supports up to 2560 x 1440 WQHD resolutions including DVI-dual link, HDMI and DisplayPort, some graphics cards only support up to 1920 x 1200 resolution content transmission via HDMI."

I have connected the iMac to the Asus screen using a mini display port to dual DVI cable. The DVI end with 24 pins is plugged into the ASUS DVI port.

No matter what I try, (menus, rest, restart, re-boot etc), I can't make the ASUS disply in high resolution 2560X1440. The Apple System Preferences ==>Displays say the only resolutions available are as follows: 1920X1080, 1600X900, 1360X768, and 1280X720.

Does anyone know of any way to get the higher resolution working?