I installed Windows 7 RC last night and my dual monitor support was not working. I queried a few people at my employer and Anthony Handley pointed me in the right direction.

If you’re using a Dell D630 laptop and you’re running Windows 7 RC and need a video driver to support dual monitors, we’ve found this one to work out quite well:

Download the driver on Dell.com

Notes: The install requires a reboot, but afterwards the dual monitor support works without an issue.