I've been having a similar problem but mine seems to run a bit deeper.
I am using the windows 7 release candidate and I have an ATI Radeon 4770 graphics card (I think this may be the problem).
I cannot get the computer to even acknowledge the existence of a second monitor; when I go to 'display properties' it just acknowledges the larger of the two, and treats both monitors as if they were the same one.
I tried disconnecting the larger of the two, and it didn't seem to notice anything had changed.
I've gone to device manager and tried to update my display adapters; it just has 'standard VGA graphics adapter' and seems to think everything is properly up to date.
Everything worked seamlessly with Vista Ultimate 64-bit and I can't even get my dual monitor set up going on this new OS.