virtualbox.org

Does anyone know if it's possible to run a VM fullscreen, and have it span across several monitors in Windows XP/7?

for example, i have a 3 monitor desktop, and I'd like to have a virtual machine take up all 3 screens instead of just one.

If you can make a window maximize across multiple monitors, you can do this, yes. Else you need to stay out of full screen and stretch the window to fit the whole three monitors. To get the most screen real estate, use a different front end for your VM, like VBoxSDL instead of just VirtualBox. Check the manual for other optional front ends. For now, it's not a VB limitation, but your OS's.