Just to reiterate:
VirtualBox: I was not able to get VirtualBox working (host: XP/SP3/32-bit, guest: XP/SP3/32-bit). I configured extra monitors as per the help file but only got black screens for the extra virtual monitors. Going into the Display Properties in the guest OS did not show any sign that multiple monitors were present or detected. Tested with between two and four monitors along with 2D and 3D options enabled and disabled.
VMWare: I was able to get this working with up to eight virtual monitors (host: XP/SP3/32-bit, guest: Win7/32-bit). After setting it up for multiple virtual monitors, I did have to go into the Screen Resolution applet under Windows 7, add the new monitors, and then extend them as part of the desktop. Typical stuff, really, when dealing with multiple real monitors. It worked a treat, too. I was even able to identify a multi-monitor related bug in one of my programs and fix it. Very very cool.
Also, VirtualBox seems to handle each virtual monitor in its own separate window while VMWare keeps them all within one, potentially huge, window. Anybody want screenshots of what I'm talking about?