Well we found out something is wrong with our cat. Or should I say that something else is wrong with the cat since he has already been treated for worms and some minor flea troubles.
Milo had an accident last week. It was all the more momentous because he had it on me while I was half asleep in bed. We had purchased a new litter box a few days before, it was covered and I assumed this was our cat telling to us that he didn't like the new way of doing things. We took the lid off the box and haven't had any more trouble. Amybear, being a better kitty parent than I am, though he might being having urinary tract trouble in addition to trouble with the litter box.
A few days later I went with her to our vet, who is an interesting guy. He just seems to spend a lot of time around animals and perhaps not enough time around people. When we told him what was going on, he suggested that there might be urinary tract trouble and proceeded to feel up the cat to see whether he could provide a urine sample. Milo hadn't gone in a while so the vet took him into the next room and squeezed the piss out of him. Literally.
When the lab results came back, it turns out that Milo does indeed have a moderate problem with crystals in his urine. Sort of like kitty kidney stones, only not as severe. We're in the process of switching him onto a special diet and Amybear is looking into a new water dish or other product to get his fluid intake up.
UPDATE: Amybear has a slightly more up-to-date take on the story at the cat blog.