I also have a dual monitor set up and experience white screens periodically. The Genius bar replaced my hard drive but that didn't help. Actually, it made things worse since they didn't know to tell me special things to do to backup outlook and I lost my virtual box info as well - time machine is not perfect.
I was wondering if there is any difference based on whether you use a VGA or a digital interface?
I have been experiencing the same problem with my Macbook Pro, 13", but my boyfriend has the same computer and uses dual monitors all the time with no issues.
We both have the same "About this Mac" Graphics/ Displays descriptions:
NVIDIA GeForce 320M:
Chipset Model: NVIDIA GeForce 320M
VRAM (Total): 256 MB
Vendor: NVIDIA (0x10de)
Device ID: 0x08a0
Revision ID: 0x00a2
ROM Revision: 3533
Resolution: 1280 x 800
Pixel Depth: 32-Bit Color (ARGB8888)
Main Display: Yes
Status: No Display Connected
Had the same problem with a 2010 13" MBP. Froze after a few minutes when hooked up to certain external displays. Hard to hard reboot, etc.
Had the motherboard replaced (thank you AppleCare) and that sorted the problem. Guy at the Apple Store had never heard of the problem, couldn't reproduce it, but took my word for it.
All sorted now and very happy.