I've been having the same issue with the Samsung T240HD, same specs as your T260HD but 24" instead of 26". You can see the post I made on Ubuntu's forums here
First thing you should try I think, is to hook up an analog cable instead of a DVI one. That didn't get me to 1920x1200, but it might work for you. See here
also (FAQ item #41).
Despite what my user name says, I am now using a 8800GTS
Am trying to get this to work with the 180.44 drivers and on Ubuntu 9.04.