My laptop has been connected to my TV (Sony Bravia KDL-52EX700) via HDMI for some time with no detection problems.
I recently got an HDMI cable long enough to permanently connect my desktop computer to the TV. Upon first plugging it in, the TV was detected in display settings just fine.
I unplugged the TV from the computer to hook another monitor up to my computer via DVI which required me to hook my main monitor up via HDMI, both worked fine.
I unplugged the second monitor from my computer, plugged my main monitor back into DVI and hooked the TV back up via HDMI. The TV wasn't detected in display settings. I tried variations of plugging the HDMI cord into different HDMI inputs on the TV, using different HDMI cords and also trying the old laptop. Neither computer would detect the TV under various conditions.
I've tried to uninstall my monitor, plug only the TV in and boot up the computer to see if it will auto-install via the wizard, but nothing ever happened. As a side note, my video card is a Radeon 7950 with current drivers (although since the laptop doesn't detect the TV either, this shouldn't be the problem).
Try disconnecting the cable at the computer. I have seen HDMI cables are sometimes finnicky. You might also have issues with drivers or settings for the video card. Maybe it is set up to clone. Try rebooting a couple of times and turning it off and on without the cable attached.
I've tried combinations of rebooting and using different HDMI cables, that sort of thing. The weird thing is, it's happening on two computers all of a sudden so it can't be the GPU on my desktop. The only thing I can think of is it's something with the TV, although other stuff works through HDMI (xbox 360, blu ray player). Sigh, I'm lost.