This is one of those “blog about it in case I need to remember how to do it later” kinda posts that I love so much. :)
I wanted to install FreeBSD on VirtualBox in order to run a few compatibility tests on an upstream package, but installing from the (64-bit) DVD ISO using the default VirtualBox settings (as of 4.1.18) resulted in what looks like a kernel panic. Not being particularly familiar with BSD, a bit of Googling was in order.
All you need to do is shut the VM down & open up its settings. Change your System → Motherboard chipset from PIIX3 to ICH9, then start it back up again with the DVD ISO hooked up to your virtual DVD drive as you normally would. The install process should be pretty straightforward from there.
This suggestion was hidden away in this forum post, but they also suggest changing your IDE Controller type. This was problematic for me & led to an error (“mounting from cd9660 failed with error 19″) while trying to mount the installation media. Instead, I booted with my IDE Controller type set to PIIX4 (I think this is the default) & everything worked great.