Punctuating question tags: A question mark is always required, isn't it. (Well, isn't it?)

The Chicago Manual of Style, 15th Edition (2003) has this entry under "Exclamation Point":

6.77 Exclamation rather than question. A question that is essentially an exclamation usually ends with an exclamation point.

How could you possibly believe that!

When will I ever learn!

If we take this guidance seriously, it seems to me, then for like reasons we ought to find it acceptable for a question that is essentially a statement to end with a period.

Consider this lyric from Lisa Germano's song "Bad Attitude": "But if life was easy, you wouldn't learn anything, now would you." I certainly wouldn't criticize a writer for complying with the standard approach of ending that statement with a question mark—and in fact I believe that Ms. Germano does use that punctuation. Nevertheless, given that her more-speaking-than-singing voice drops by more than an octave between "now" and "would you," I wouldn't think it misleading to end the sentence with a period.

In addition, Chicago 15 has an entry under "Question Mark" for what it calls "courtesy questions":

6.74 Courtesy question. A request courteously disguised as a question does not require a question mark.

Would you kindly respond by March 1.

Will the audience please rise.

That gives us two instances in which a widely influential U.S. reference work endorses using punctuation other than a question mark to end a phrase otherwise structured as a question.

I tend to agree with Robusto that strict adherence to the rule requiring all statements that are laid out in a form that would normally identify them as questions to end in question marks prevents writers from indicating, as they otherwise might, whether the intonation of the speaker's voice is rising or falling. The bad aspect of any widespread effort to differentiate intonation by punctuation is that it invites countless instances where authors choose punctuation mismatched to their intended intonation, and readers—newly alert for clues to intonation in the punctuation—consequently misinterpret the sentence.

I don't normally answer my own questions, but in this case I feel compelled to do so. FumbleFingers' answer — the only answer this question has received* — while well-argued and not incorrect, feels like the answer of someone who is faced with a problem he recognizes but cannot solve, and so falls back on whatever has served in the past: in this case, the "standard rules of punctuation."

I've thought about this for a long time, and I am coming to the opinion that "standard rules of punctuation" are a fluid concept, changing with the rest of language. Certainly no one would suggest that the punctuation rules of the 18th or 19th centuries would apply today. The meaning of entire statements in the U.S. Constitution, for example, would seem to turn on archaic styles of usage for commas. Just look at the Second Amendment:

A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.

By today's standards, there are too many commas. The extra commas have created a storm of controversy. We would not write the same sentence the same way today. Punctuation has moved on.

I am going to suggest, then, that punctuation may be used (to some, abused) as a matter of expression. Writers have been doing this for many years. Normally we would suggest that dialogue in text, say, should be governed by "standard rules of punctuation"; the statements made by characters are surrounded by quote marks, and there are rules governing what is done with other punctuation inside and outside the quotes. But then we look at works by James Joyce or William Gaddis, and we see that some authors, at least, decided that an em dash was a better way to set off instances of dialogue. Here is a fragment from Joyce's novel A Portrait of the Artist as a Young Man:

Stephen had answered: Stephen Dedalus.
Then Nasty Roche had said:
—What kind of a name is that?
And when Stephen had not been able to answer Nasty
Roche had asked:
—What is your father?
Stephen had answered:
—A gentleman.
Then Nasty Roche had asked:
—Is he a magistrate?

It is worth noting that Joyce doesn't even use an em dash in the first line. He uses a colon instead. Similarly, Faulkner tried to move English forward by eliminating apostrophes from some of the contractions (notably, dont, wont, cant and aint). Now, you may argue that none of these treatments caught on, but that does not negate the work of these authors, nor call into question their right to make their own rules in the pursuit of expression.

At this point I have to remind everyone about one of the conditions I cited in my question above:

But in written form, especially dialogue, it feels to me that question tags meant to be spoken with a falling intonation might get by with just a period

So — again, reminding everyone that we are talking about writing as an expressive medium, and that nobody has addressed this aspect of the question — I am going to conclude that using a period instead of a question mark at the end of a question tag may be construed as a matter of stylistic expression. I wouldn't do that in an academic paper, or a scientific treatise, of course, but we are much less likely to encounter dialogue in those venues anyway.

^{* As of the original posting date; obviously that's changed now.}

While it's ultimately a matter of style and preference, I think the case for the period is very strong. The period makes the writing clearer without becoming a distraction, and that's what good punctuation should do.

Using italics to distinguish the two kinds of tag questions works, but adding more punctuation creates more distraction.

The only real argument against the period that I'm aware of is that the punctuation "error" becomes a distraction. But in my experience this is not the case -- readers get used to it very quickly.

I'm not sure it counts as a "definitive answer", but so far as I'm concerned punctuation can't be used to differentiate OP's two intonations (and hence, meanings).

The standard rules of punctuation require the question mark to follow all constructions framed as questions, though style guides generally make exceptions for either/both of these types:

The teacher was waiting for them when they got back and was she mad! (really an exclamation).
Miss Kate, will you take a seat over there in your chair again. (courtesy question/invitation/order).

It may be worth noting that by introducing an italic font as well as punctuation, some other interpretations can be implied...

"You didn't leave the dog in the car. Did you?"

...which with no other context, implies the speaker is addressing two people - stating/confirming that the first didn't leave the dog in the car, and asking the second whether they did.

"You didn't leave the dog in the car, did you?"

...which implies OP's "former" case (speaker is concerned/incredulous), but shifts the element of surprise to the fact that the dog might have been left in the car, rather than somewhere else. One could just as easily italicise/emphasise the word "dog" (surprise that we're talking about the dog, rather than, say, the baby).

Those contrived contexts are probably only two of many alternative meanings that can be conveyed by how the text is written, but the level of diferentiation OP seeks simply doesn't exist.