Pixie, the Great Pyrennes we got from the flea market in April - has a condition (which in humans is called 'trismus'), which basically means she can't open her mouth more than a little way, about a quarter of normal. Masticatory muscle myositis or another neurologic condition is possible, as well. We didn't notice it until yesterday when she couldn't take a biscuit. She has been eating through the side of her mouth and drinking water -- and didn't seem unhappy until the biscuit was offered and she couldn't take it.
She is at the vet right now, they will sedate and x-ray her trying to figure out what it is. Our first thought was tetanus / lockjaw, but she doesn't seem sick any other way. Even this morning she was playing with the puppies and running around the yard without any other symptoms.
Update 2 years later: Our dog still has a mostly locked jaw. They never could get it any better. She has lost a lot of weight but she learned how to eat through the bit of space she does have -- and does pretty well when the other dogs leave her alone with the dish to herself. My husband still gives her bits of fat and good pieces of meat sometimes to make sure she is getting calories. She seems to be okay with things as they are - plays some, wrestles the puppy some, never exuberantly but at least she isn't always acting like she is in pain. She still tries to bark at the deer and squirrels - but it sounds like somebody has a bag over her nose. We wish luck to all those who have commented with ill pups - hope your story is at least as happy or happier than this one.
Final Update 3 years, 2 months from original posting date: Our girl passed away today - we did as much as we could for her, for so long... She didn't even let us know she was bad off at the end. She was being herself. I think her cheerful 'what now?' attitude was a large part of her surviving this long. Still will wish hope for others who have had this happen to their animals -- take care of the while you can and let them enjoy what they can while they are here.