Still no adapters released, so we can't say for sure.
But technically it's possible. HDMI is a digital signal and Tunderbolt is designed to accept/transfer everything.
Apple mentions HDMI on their Thunderbolt page:
http://www.apple.com/benl/thunderbolt/
But says "...just use an existing adapter". Since Thunderbolt has the same port as mini-displayport, they could refer to the HDMI to mini-displayport adapters

which costs 150 dollars

...
But here the good news: Intel showed a a list of manufacturers which will make Thunderbolt products. 3 companies catch our eye: Avid, Apogee and Blackmagic. 3 companies which have HDMI adapters/converters. Ofcourse we don't know for sure they will release HDMI to Thunderbolt adapters, but since they will release Thunderbolt devices and are in the branche... We can just hope...
And there still might be some other companies who want to realise this.
Like the companies who sell the Mini-displayport to HDMI adapters right now like Kanex. They still depend on a seperate power source, but since Thunderbolt is able to give enough power, this might be a promissing upgrade for their mini-displayport to HDMI adapters!
As conclusion: There probably will come HDMI to Thunderbolt adapters, but when? And for how much?