Hi folks.
There was a similar discussion in the SketchUp Help Group.
Here is what I answered:
Make sure that the videocard of your Mac is able to use the optimum resolution of the 30" display that is of 2560 x 1600 pixels as per this page on Apple Web site:
http://support.apple.com/kb/SP79
Normally, the manual that comes with your Mac shall have its technical specifications and these shall give you the maximum resolution of your graphic card.
Since you don't give the screen size of your Macbook Pro, it is difficult to guess what one it is but I suppose it is 17" since you can drive a 23" cinema display and you gave the video card make and model number as well as the amount of VRAM (video RAM).
This page:
http://support.apple.com/kb/SP546
State that the NVidia 9600M video card with 512Megs VRAM can support the resolution of the 30" cinema display combined with the 1920x1200 native resolution of the built in display.
See this paragraph:
----- START OF EXCEPRT -----
"Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 2560 by 1600 pixels on an external display, both at millions of colors"
----- END OF EXCERPT -----
All this info comes from the Support section of the Apple Web site.
Just ideas.