How to Force choose a Video Resolution

I have a MBP, 17", 2.4 GHZ, 4GB , with the GeForce 8600M GT Graphics Card. If I hook it up to a Plasma using a DVI to HDMI cable, it recognizes the device it's connected to and will offer the available resolutions, including 1080p. I need to hook it up to an AJA IO HD Kona device, and when I do it recognizes the device, but I cannot choose the 1080p resolution. It offers the 1920 x 1080, but I need for it to have the actual 1080p resolution output.

Any advice/software/hardware configurations out there where I can force choose a resolution that isn't on the available list chosen for me for the device connected?

Uh.... 1080p is 1920x1080. Am I misunderstanding something here?