First things first - PFsense project is AWESOME!!!
VERY BIG THANK YOU to everybody who has been and still is (as well as will be) involved with this project and all the add-on components like HAVP, SNORT, SQUID, PFblocker, and many others.
This post only applies to setting up Pfsense 2.0.2-RELEASE (i386) with havp-0.91_1 HTTP Antivirus Proxy, and may or may not apply to other released versions... No SQUID installed!
After installing HAVP package you may realize it's not working in Transparent mode without SQUID caching proxy installed.
The workaround is to add new firewall NAT rule which will forward all local LAN HTTP requests to internal HAVP proxy port (default port is 3125) on the box.
After saving new port forwarding rule you should see corresponding (linked) entries under NAT and Firewall tables.
To test transparent antivirus proxy configuration open the web browser and try to download fake malicious file from EICAR site... If your transparent mode is working correctly you should see HAVP block page similar to one below.
If you need to modify HAVP reporting html templates they are located under:
/usr/local/share/examples/havp/templates/en (your location may vary)