I'm running Ubuntu 12.04 LTS on a Dell XPS L502X and when I connect the HDMI cable from the TV to the computer Ubuntu detects nothing at all, It works perfectly fine in Windows 7 though. I've already went to the Display control, plugged and unplugged the TV, clicked the Detect Displays button, and nothing. Do I have to activate the HDMI output or something? Because I used another computer with a VGA output and it worked perfectly.
On 12.04 the Nvidia cards are not recognized in the display menu. You have to use Nvidia-Xserver-settings to do that.
Type Nvidia in the dash and open Nvidia-Xserver-settings.
Click on X server display configuration and then on detect displays. There you will find many more tweak options. Once ready click on apply and you should be all set.
I have the exact some model, i had this problem to, and i have tried everything you possibly can do and nothing works.
My workaround was to buy a mini-display to hdmi that looks like this
Every Mac laptop's has this and i really don't know why Dell has it but it does and it works well, you should be able to get one at your computer store if not try here
I asked this question earlier this year and this was my solution. You can find the question here