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This page details the ongoing effort of porting VirtualBox to FreeBSD.
There is also a chapter about VirtualBox on FreeBSD in the Handbook.
Tuning Guide
Network
If you transfer big chunks over the network or have heavy network load netgraph probably runs out of memory.
/boot/loader.conf on the Host:
# Increase netgraph memory size for heavy VirtualBox network load net.graph.maxdata=65536
Harddisks
VirtualBox can emulate ATA and SATA controllers but defaults to ATA for FreeBSD guests. FreeBSD also supports SATA and AHCI and performance is a lot better if you have AHCI enabled on the host and the guest. Since FreeBSD 9.0 the new CAM infrastructure is enabled per default so you already see your devices as /dev/ada0. For FreeBSD 8.x you need to explicitly enable it but beware - your device names will change from /dev/ad0 to /dev/ada0 so you probably have to modify your /etc/fstab too.
/boot/loader.conf for FreeBSD 8.x on the Host and Guest:
ahci_load="YES"
VirtualBox: TODO: how to enable AHCI on SATA controller