Lazywebs, here is an answer. Having problems with a black/blank/undetected screen on your HDMI out in Ubuntu? Using x86_64 and Nvidia? Here's two tricks I discovered. I'm failing at being a good netizen and digging up the tens of various threads on similar problems, creating/remembering/resetting my accounts and posting my solution there, but hopefully the magic of google will lead fellow lost souls here.
If none of this makes any sense, get back to your eggnog.
In a (successful!) attempt to hook my Lenovo y550 laptop running Ubuntu 10/Maverick Meercat to my parents 720p HD TV via the HDMI port, I encountered an unusual (this used to work fine!) number of errors, and a lot of dead ends. The first being finding an HDMI cable out here. After a grueling run through the various big-box stores, selling the damned thing for $40 and waay up (they cost under $6 on Amazon.com), I peeked into the neighborhood A/V shop near my cousins' (two cousins, one shop) western-ware store. He gave me a fair price for an unbagged, ~1m cable: free. I love small town economies where customer referral and satisfaction are more important that 10 cents of profit.
OK, back to the geekiness. I had to grab the latest drivers from nvidia (260.19.29), drop out of X and stop gdm, install those drivers, and reboot. This got me to detecting the TV, but no matter what configuration I tried, I just couldn't activate it. In desperation, I moved on to using the nvidia-settings tool to start mucking with my xorg.conf file, and it threw a permissions error. I backed down but realized that this might be the core problem. Up comes a terminal to sudo nvidia-settings to run it with root permissions, and magically the screen started working when I re-applied the TwinView settings.
That's all. From there on out, it was smooth sailing.