If you setup Backtrack 4 in a way that you use a normal non-root user for everyday work you'll notice that most tools require you to run them as root. This is no problem, as you can always fire up a terminal, "su root" and fire away or simply use gksudo for graphical apps. I noticed that whenever wireshark is started via terminal while logged in as my normal user, it would start up properly, but not show any networking interfaces. So I did a "su root" in terminal and fired up wireshark again as root only to be greeted by a warning like this:

Gtk-WARNING **: cannot open display: 0:0
I was able to fix this by issuing the following command in a terminal as my normal user:

xhost +local: