Damn i also have used a lot of time to get my UT2003 server running.
I just want to share the things i have tried, and maybe get some help with the last thing i need to do to get i totally working.
My setup is..
Windows XP on a Intel 4 and so on....
1 Zyxel Router 650 Serie.
I have as all of you open port 7777,7778............
BehindNAT=True in server ini file..... and so on
When i am starting my server, i also "did have" the "PING = N/A" in UT server browser.
BUT last night after reading some FAQ at Zyxel home page i found a command called " IP NAT LOOPBACK" They described it -- being able to look at web server from LAN.
After setting IP NAT LOOPBACK to ON.
I WAS able in UT Server Browser, to se my server with PING and all the information normally written when higlighting a server. 
The only problem i have now is, - when trying to connect i only get the black conneting picture " .....\\IP.IP.IP.IP and nothing more happens.
So if anyone have the gold for this i would be :p
Love, Peace and im sry for my english. :sigh: :sigh: