This is another problem I've been having with my HTPC besides the TV tuner card issue discussed in another topic. I bought a nice new full HDTV - Toshiba 46" LCD - earlier this year. For my HTPC I bought a Sapphire HD 3450 Video Card based on advice from friends.
Despite my best efforts I have not been able to get a picture through the DVI port to the TV using a DVI to HDMI cable at any resolution let alone 1080p even though the card is supposed to do this. The only way I'm getting a picture is via a VGA cable using either the VGA port or the DVI-VGA convertor supplied with the card. Of course this does not give me 1080p resolution but at least it works. I have three other PC's including a laptop with DVI out. All the other PC's use NVIDIA hardware rather than ATi and I can get a picture on my HDTV from all the PC's with NVIDIA graphics using the DVI-HDMI cable so the cable works. Its just the ATi card that doesn't.
After asking questions of my friends I finally found someone who seemed to know what the problem is. It appears that I need one of these - http://www.theinquirer.net/en/inquirer/news/2007/05/03/atis-r600-hdmi-dongle-revealed. The newer ATi cards come with them standard but my card was bought in February and didn't come with one. I can't get a hold of one to test it so I can't say with certainty that it will solve my problem. This same person has also recommended I buy a NVIDIA 9600GT card for my HTPC. What advice can you lot here offer me.