@DrewDad,
Thanks for the responses.
Good to know I might get an engineering type to take a look on the cheap. My cousin is a civil engineer who's worked on phenomenally wet and unsturdy surfaces, but the airfare from California would hurt.
The house is 63 years old and is shockingly level throughout, so I very much doubt that there is a structural issue that is going to affect the house itself. The problem doesn't seem to be with the basement wall, but with the floor settling downward away from the wall.
It hadn't occurred to me that the basement floor might be younger than the house. This is a strong possibility, I think. A chimney and fireplace were removed sometime in the past 25 years (based on a conversation I had with a former resident). The dormer was added sometime after building and before the mid-70s. The base of the chimney -- appeared to function as some sort of old-school basement furnace or something is still in the basement. Why it wasn't removed as well I have no idea.
I do wonder about the water situation. The neighborhood apparently was flood prone until the 70s, when the city installed an underground drainage sluice along the side street. This is about 25 feet from the side of the house, though, and about 40 to 45 feet from the problem area.
I'm inclined to blame the improper drainage from the dormer first and foremost. I regraded away from the house and put in window well last summer -- maybe I'll just go with new gutters this year and see if this remedies the problem, and then just patch the crack if it does...
The real question to my mind is the rate of settling. If it's 2 mm every 60 years, I'm not too chuffed about the resale value in 10. If it's 2 mm a year, there's clearly a serious problem.
For what it's worth, most houses in Madison this age seem to have seeping spots in the basement. This is by the far the dryest basement I've lived with here, but, you know, I own this one...