For the record, the problem was the water pump. The main bearing had developed a leak. I got the old part back just to see if I could visually detect any problems:

First, this water pump seems to be built very ruggedly. The impeller blades are heavy duty and the general appearance of casting and machining is almost impressive.

Unfortunately, the shaft on my pump apparantly had shifted axially in the outboard direction, thereby causing the impeller blades to "scrape" against the housing. There is a distinct groove in the housing.

Also note the expense required to replace the water pump. The part is about $170, plus four hours labor, antifreeze, etc., and the bill comes to over $500 quickly! Ouch!