The other post was closed, so I'll reply to this one.
I have several linux distros and FreeBSD installed on one system, although I have the BSD install on a different hard drive than the linux distros.
In any case, the key for me was to tell BSD not to install it's boot loader onto the MBR of your main booting disk. After that you can just use grub to load it by creating an entry like this:
title FreeBSD 5.2.1
That tells Grub to pass off loading to whatever it finds on disk 0, partition 4, slice a.