I have Ubuntu 10.04 installed as my primary operating system, and I installed PC BSD in a different partition: /dev/sda4 without installing it's boot loader. I figured out that I need to edit /etc/grub.d/40_custom to add an entry for PC-BSD. So far, nothing seems to work, though.
EDIT: this sort of works, but doesn't fully boot the OS, it then asks me for the MOUNTROOT partition.
menuentry "PC-BSD 8.1" { insmod ufs2 set root=(hd0,4) kfreebsd /boot/kernel/kernel } The selected answer below is correct. If you are dual-booting with Linux I suggest NOT installing the PC-BSD bootloader as the documentation suggests, unless you enjoy pain.