I was in the middle of writing a detailed guide when the system froze... damn.
Here is the short version abut getting Intel 4965agn running on PCBSD 7.0.2:
- get the FreeBSD source if you don't have it
- download the drivers from http://sites.google.com/site/bsdgooch/files
- apply the diff file in /usr and copy the src/ folder from the archive to /usr as well
- compile the iwn and iwnfw modules
- copy the 2 .ko files (iwnfw.ko and if_iwn.ko) to /boot/kernel
- add the following line to /boot/loader.conf:
Reboot your system. If everything is fine, you should see the iwn0 interface. You can configure it with the network configuration "wizard" in KDE.