When I switched my video card after the capacitors burnt out on the old one, I found I could no longer do 1366×768 video on a Samsung LN32A450C. Most frustrating was the fact that I’d see the video for about four seconds, before the TV decided it didn’t want to display it and showed “Mode not supported”.
Apparently, this is a common problem with Samsung TVs – and cvt was no help.
Luckily, I found this post – so here is the modeline for the Samsung LN32A450C
Modeline "1360x768" 85.500 1360 1440 1552 1792 768 771 777 795 +hsync +vsync
I just had to add that to my Monitor section of /etc/X11/xorg.conf and the autodetection on Ubuntu 9.04 did the rest. This is actually 1360×768, not the specified 1366×768 that Samsung is supposed to support – but I don’t miss the few pixels on either side.