I'm pleased to note that the letter I drafted has been well-received:
I've reproduced my letter below:
Thank you for your submission. I regret to say that it does not meet our needs at our time.
I know that many people tend to read between the lines of form letters such as this one, searching for the "real" reason why their submission was turned down. To be perfectly honest, saying that a submission does not meet our needs is simply that -- a statement that what you sent is simply not what we're looking for.
I regret the necessity of sending you this form letter, but please understand that I receive well over five hundred (or whatever) submissions a month. If agents spent our time crafting personal notes when rejecting material, we'd never have time to accept material -- which is the eventual goal on both our sides.
Good luck with this submission elsewhere. If it is accepted by someone else, feel free to gloat.