I was getting an error similar to the one here:
http://forums.macosxhints.com/archive/index.php/t-38052.html
As soon as webspy would catch a url and pass it to the browser to be opened, it would fail with this error. This is because webspy is running within the X11 environment, and is outputting to the browser attached the the same display (echo $DISPLAY for where you are outputting to, this will be a UNIX socket or a TCP port).
The easiest way to deal with this is to install an X11 browser and launch it from within the environment. There is a darwin ports Firefox-x11 2.0 build that I used just fine.
sudo port install firefox-x11
Let it build and install, I had to make a couple of small corrections during the install for gtk to work right, but ports was verbose with instructions when problems came up. I also had a problem with fragrouter misbehaving with libnet_write(). Instead of using that, I suggest you just turn on ipforwarding with the following:
sudo sysctl -w net.inet.ip.forwarding=1
That should do it, combined with arpspoof, you should be all set. Any snarfed URLs will be redirected to your X11 Firefox.