I'm having the same issue with a NVidia GeForce 9400 GT. The PCI express card has three ports in the back - HDMI, VGA and DVI.
I'm using Windows 7 - 64bits. and the latest Nvidia Driver available. (257.21 - 2010.06.15)
The HDMI port is not working when I connect it to a TV. Windows does not recognize it or make any sound when I plug it.
The VGA port works fine with the Monitor, the DVI port also works fine. I had tested the TV by connecting it to a DVI port with a DVI-HDMI adapter and worked fine. So the cable and the TV is working fine.
Any tips will be appreciated.