I had this same issue with squid 2.7.9. This worked for me:
Set squid proxy to listen on port 3129 (or any port you choose, the GUI wouldn't allow me to leave it blank)
Add custom option: http_port 3128 transparent
Port forward on LAN:
Traffic TCP Src * Srcport * Dest * Destport HTTP(80) TargetIP pfsensebox IP Targetport 3128
My guess is that on the GUI without the transparent box checked, squid was not operating transparently on port 3128 until specifically defined to do so.
Unfortunately my ultimate goal was to use this rule to apply limiters to the traffic but apparently there is a bug with limiters and squid in transparent mode that I can't seem to get around!