I too haven't managed yet, because of the X security on the IT.

What I managed to do is launch an app and display it on my PC but _NOT_ tunneled (so all passing unencrypted..)
Using kdm and debian
First the local Xserver has to listen for incoming connections. By default in Debian this is hardcoded so that it does not listen, for security reasons.
vi /etc/kde3/kdm/kdmrc
Change
ServerArgsLocal=-nolisten tcp
to
ServerArgsLocal=
Restart X windows (ctrlAltBackSpace)
then login and allow people to connect to your DISPLAY:
xhost +

then log in to your IT:
ssh me@192.168.1.5

set DISPLAY variable
export DISPLAY=linux_ip:0.0

and start an application that is linked to xlib (which uses the DISPLAY variable)
xchat

Pops!

Now the real X11 forwarding would be xchat would connect to localhost:10.0 (IT:10.0) tunneled to PC:0.0
But no xauth so connection to localhost:10.0 doesn't work (due to missing xauth yes)
Maybe as user "user" it does work.
I'll try again later and post here