Just last night I was messing with VirtualBox, and according to their website OpenBSD should load fine. Surprise, surprise, OpenBSD bombed on VirtualBox during hard drive setup.
My experience has been similar with FreeBSD in Virtualization. I didn't bother checking FreeBSD in VirtualBox because the site specifically states that FreeBSD 6.2 has known issues and doesn't really work, but I've yet to get FreeBSD/OpenBSD working in virtualization (haven't tried a static sized image for OpenBSD in VirtualBox, though...I'll try that tonight after work).
I've tried Bochs, Qemu, and VMWare for virtualization (and VirtualBox now as of last night), and I've had them all fail (Qemu is really fast and relatively stable) at one point or another.