Get pfSense-1.2.2-VM.zip (VMWare image) and pfSense-1.2.2-LiveCD-Installer.iso (from one of existing mirrors).

Extract VM.zip and import VM disk into VirtualBox using File / Virtual Media Manager. Set up your new machine to boot from downloaded LiveCD .iso and use newly added hard disk (imported from VMWare image).

Since you are booting into LiveCD set up your LAN/WAN interfaces and go to shell (option 8). You will have to check disk using fsck /dev/ad0s1a, mount it using mount -o rw /mnt and edit /mnt/etc/fstab to change /dev/da0s1X to /dev/ad0s1X.

Save you changes to fstab and halt the system. Remove LiveCD from you machine settings and you should be good to boot pfSense on VirtualBox.

-su-2.05b# uname -a
FreeBSD xxx 5.4-RELEASE FreeBSD 5.4-RELEASE #1: Sat Aug 27 18:17:42 CEST 2005     root@xxx:/usr/obj/usr/src/sys/PF  i386

-su-2.05b# uptime
8:37PM  up 617 days,  4:46, 1 user, load averages: 0.00, 0.02, 0.00

I guess this would have been at least doubled if they haven't had to migrate equipment to the new server room some time last year...

As they say - if it ain't broke (and exposed directly to the net) - don't fix it!