After I got back from tonight's readthrough, I let the cat out; Grendel was meowing and complaining at the door, as per usual. What wasn't usual was that he meowed to get back in about ten minutes later, at the front door. Usually, he galavants around a bit longer and comes to the back. When I opened the door, he literally dragged himself in. He couldn't use his hind legs at all.
efbq got the number of our vet, who has a veterinary hospital in Ellicott City take his off-hours emergencies. An hour later, she's gotten the cat there. I just got the call from DW that apparently it was a stroke, and his prognosis is quite poor. He's staying the night at the animal hospital, and in the morning we're getting him to our regular vet and it appears we'll have some difficult decisions to make.
EDIT #1: He's back home now, still not in a good way but seems to be a little bit better now that he's home (and rather sedated). Kid-1 has had a right good cry, and decided to go to school - she felt it was too hard for her to stay home. Kid-2 is staying at home, and is kind of trying to convince herself that he'll still be okay. We're taking him to the vet's office in about half an hour.
EDIT #2: The cat is at our regular vets now, and they have dosed him with something that is designed to break up blood clots, on the chance that it's thrombosis. At this point it's a waiting game. We'll know within 24 hours from the dose whether or not it had an effect.