I'm running OS X 10.8.5 and VirtualBox 4.3.10.
My guest OS is Windows 7 32-bit and it set to have a single monitor in the settings.
When I put my guest into full screen mode on my external display, it properly goes into full screen on the second display, but the first display no longer shows my OS X windows and instead is blank - well, not technically blank, but the cross-hatched gray and blank pattern.
I'd like the primary display to continue displaying OS X windows and the secondary display to show my VM in full screen. I seem to recall that this used to work back in 4.2.x at some point, though I just switched back to using VirtualBox after 8 months of using another product. In other VM products, I'd look for something like "Use all displays" but there doesn't seem to be anything like that, or I'm missing it.