Ok, now that I think about it, I will soften my view just a little, as I think I may have contradicted myself. I did say it is possible to make a different sound for mmm and nnn, although only slightly different. Thus, it is possible to differentiate them when put together. I would still say that the difference is only barely perceptible, and then, only when great effort is taken to create two distinct sounds. It is no wonder that the n in question is treated as silent. It's almost impossible to make it sound any other way. (see? I added the word almost!)

now I KNOW this will start an argument, but I am going to posit that it is IMPOSSIBLE for ANYONE to say autumn or column in such a way as to pronounce the "n" at the end. In fact, I would claim the sounds "mmmmmmm" and "nnnnnnnn" are indistinguishable from each other. In order to tell the difference between "m" and ''n", they must be either preceded or proceded by a vowel.
In order to pronounce autumn and make the "n" sound clear, you would have to pronounce it "au-tum-NUH, putting a schwa at the end. You could choose to do so, but I wouldn't say that that is a normal pronunciation by any standard. In effect, you would have to add an extra syllable.
Now, I know that a bunch of you are going to say "I can tell the difference between 'mmmm' and 'nnnn'", but I say you can't. Here's a test. Try saying "mmmmnnnnmmmmnnnnmmmmnnnn" continuously. Yes, you can make it two distinct sounds, but you can just as easily make it indistinquishable. In any cays you really can't tell which is which, especially considering the normal variation from person to person. The problem is that when you make an m or n sound, all the air is rushing out your nose and resonating in your throat. The shape of everything inside your mouth has only a slight (or possibly no) effect on the resonance. You can make any number of shapes with your tongue, teeth, jaw, etc. but they all sound basically the same. Depending on whether the back of my tongue is against the roof of my mouth, or my jaw is dropped, I can make a hundred different and distinct sounds but they all sound like mmmmmm, especially to someone else.
Now, you can tell the difference between "ummm" and "unnn" or "maaa" and "naaa", but not between just plain old "mmmm" and "nnnn".

hi Zoltar,
Are you familiar with Warner Brothers cartoons? (Bugs Bunny, Porky Pig, etc.) The phrase was originally used in one of their cartoons featuring the giant rooster character, Foghorn Leghorn and a young chicken hawk. The chicken hawk is doing something (I don't remember quite what) that Foghorn thinks is stupid or a waste of time. Foghorn turns to the audience and says something like "I say, that boy is about as sharp as a sack of wet mice."
I believe this was not a genuine expression before this, but was made up for the cartoon. It has since become a somewhat common expression. The meaning, of course, is that someone is dimwitted or stupid. The double entendre is sharp as in smart (intelligent) compared with sharp (pointy) vs. dull. Clearly a bag of furry, wet mice would not be sharp.
It's similar to another expression, "dumber than a bag of hammers", hammers being one of the simplest of tools.

Molly, check out dyske's posting below. According to the dictionary, both your grandfather and great-grandfather could be considered 1st generation. The expression is ambiguous.
Now, let's have some fun. Was the expression always ambiguous? If not, what was the original definition?:)

Jon, oi in England? Thanks for the info. My Mother-in-law is English and I hear an occasional "oi" out of her. I would always think to myself, "How odd. Why is my Episcopal Mother-in-law speaking Yiddish?" Now, you've clarified it for me:)

Brian and Michael have pretty much hit it on the head. Do note, however that I don't think it's necessarily military. It applies in all forms of radio communication. It is actually an officially documented expression for all licensed FCC radio communications, ham radio, aircraft communications, etc.

I know some will agree with me and some won't, but I was taught in school (some time ago, I might add) that the comma in question was optional. This is what I was taught and what appeared in print in my grammar textbooks throughout my entire education. I mean optional with no qualification whatsoever. Either way is completely correct.
I generally add the comma because it does usually eliminate any possible ambiguity, but I view this as purely a matter of personal preference and would never claim that the opposite view is incorrect in any way.

P.S. - In many postings on this website, I often see references to various style manuals as justification for all sorts of points of views. I would like to remind you all that, while useful to help support a particular position, style manuals are just that: STYLE manuals. They are not authoritative RULEbooks. A newspaper, magazine, or trade organization (etc.) uses one to create a uniform appearance in their writings. Ambiguous rules may be resolved in such a manual, but, that's because sometimes grammar rules ARE ambiguous. The manual doesn't claim to be right or wrong. It just establishes a common practise for the sake of consistency within the organization.

P.P.S. - Maybe I should start a new post for this, but I was taught that words like however, regardless, thus, and even the simple "but", when used in the middle of a sentence, should be preceded by a semicolon and followed by a comma. Today, it seems that, especially with "but", it is preceded with a comma with no comma afterwards. I believe that modern grammar now accepts both but prefers the simpler. Comments?

I think I have to take back my suggestion for "as wet as rain." The common expression that i was thinking about is "right as rain." Sorry, wrong dead metaphor, er, simile.

oops, I meant "having" not "have"

You certainly ask an interesting question, but I have to disagree that "...in this case both correct." In the example you gave, if you substitute the word "have", then the sentence becomes nonsensical, so you CAN tell by the context, at least in this case.

But I am curious as to why you find this so unusual. There are many thousands of homonyms, or more specifically, homophones in the engish language. There are innumerable ways that truly ambiguous sentences can be constructed. Throw in homographs, words that are spelled the same, but have different meanings and sometimes different pronunciations, and it gets really interesting. Such ambiguity is often at the basis of poems literature, even art and music. This is often exploited in jokes as well.

For a little fun, see my previous post in
http://www.painintheenglish.com/post.asp?id=524
(Owed to a Spell Chequer)