Correct, it could result in winter damage that kills some or all the flower buds. We had a similar case 5+ years ago here when temperatures stayed in the 70s thru December so the hydrangeas never went to sleep. When the cold temps finally came, it was in the form of a week of sub-freezing temperatures. A lot of shrubbery and trees got damaged. I got 2 blooms total the following Spring from all the hydrangeas.
You could try winter protecting them by creating a wire cage at least 6" wider and taller than the plant and filling it with dead leaves, hay or some other organic material. Pack the leaves real dense so they protect the ends of the stems which is where the flower buds are. Add more leaves if settling occurs by mid winter. This will not stop them from leafing out but it will protect the flower buds some... but the temps are the real problem here and there is no way to make the plants feel colder and go back to sleep. If unprotected and the freeze lasts a little time, it may not hurt the flower buds much; it may be the kick that finally gets them sleeping.
If affected, the damage may vary. Rebloomers and old wood varieties should see their (first) Spring flush affected. Old wood varieties may not bloom in 2012 or may bloom poorly compared to previous years; how much depends on the freeze. Rebloomers will still produce another flush starting in late Spring that will not be affected by this month's freeze. So you will have blooms from them; just starting later than usual (June?) but still. And finally, the ones that bloom on new wood (paniculatas, etc) will not be affected.