I still need to boot into a FreeBSD partition on my laptop to do some FreeBSD specific things now and then, which you can't do in a virtual environment.

Setting up FreeBSD to dual boot with Ubuntu (or any other Linux with grub) is simple.

1. Install FreeBSD in a primary partition (slice) and leave some space free for your Ubuntu install during the fdisk part of the installation.
2. Install Linux in remaining space, and install grub boot loader (usually the default)
3. Boot into Ubuntu as it doesn't autodetect FreeBSD partition, and edit /boot/grub/menu.lst (or grub.conf on some other distros) and add the following,

title           FreeBSD
root            (hd0,0,a)
kernel          /boot/loader