Update, it works. I retried connecting my laptop to the TV after verifying everything with a second laptop running Windows 7. This time in nvidia-settings a second monitor appeared and I could extend my desktop to the TV. But sound does not work. There is a HDMI audio device in the mixer, but I can't get sound via the TV (worked in my test with the other laptop, so connection-wise it is OK).
Update: I have a HDMI device in system-settings->multimedia->phonon. And when I click the test button the TV actually plays something which sounds like the test-melody with a lot of noise. Standard application sound is only audible on laptop speakers. Any ideas on how to fix this?
Video: Seems the TV has to be set to the input connected to the laptop. Then nvidia-settings detects the external screen and twinview can be configured.
Audio: No full solution yet, but for me following commandline worked to get audio and video via the TV set:
mplayer -ao alsa:noblock:device=hw=1.7 <videoFile>
Had to experiment with the 1.7 argument a little. First 1.3 worked, after suspending the machine and waking up again 1.7 did. (found the command here: http://www.thinkwiki.org/wiki/DisplayPort)