Due to a comedy of errors (I'm not laughing right now), it turns out that my macro lens hit the floor tile in my hallway. Frantic testing produced abnormal behavior, which
tugrik confirmed to be damage to the USM ringmotor.
The good news, as such, is that a repair shouldn't run me more than 1/5 of the price of the lens.
The bad news is that, well, let's face it -- a $520 piece of glass hit the ground and suffered damage as a result.
This is not how I wanted to start my vacation two days before my birthday.