While I don't like having to spend yet more money, at least I don't have to pay sales tax on the parts.
Update, 15:30: Dealership confirms that it's the blower fan, and quotes a price of a little over
Further Update, 16:30: They forgot to quote me the entire fan assembly including the cage that actually moves the air. Still doesn't seem too bad to me. Also, it's replacing an after-market blower with a GM part, which seems like a better thing to do.