Okay, this one's mainly for fun, hence the slightly Scrooge-like fee. 
So don't feel like you have to write an essay, but if you've got some
plausible theories (or, better yet, some facts) I'd love to hear them!

Here's the deal: We share our house with two cats.  They are young
Maine Coons (absolutely beautiful animals; big, soft and fluffy). 
When we sneeze they react in a very odd way, by gaping their jaws and
making a strangulated, warbling 'ack-ack-ack'.  The only other time
we've heard them make this noise is when they are closely watching a
fly or a moth that is just out of reach.

One of the cats reacts in this way without fail every single time..
..even if you're barely in earshot.  Sometimes it's a brief noise,
sometimes he'll 'cry' like this for a good minute or so in bursts.  If
I sneeze more that a couple of times in succession, he almost always
slinks off out the room, as though it bothers him.

The other cat never used to react this way, but is doing so more and
more (they are just over a year old).  Something else I can tell you
is that he only seems to do it if he's dozing off, or if he's actually
asleep!

There are three of us living with these cats and they only do this
when the men sneeze.  When our female housemate sneezes: absolutely
nothing!  Is it something to do with frequency?  Is it bothering them,
or does it just make their nervous system tingle?  Do they think we're
hissing at them?  A brief search suggests our cats are not unique. 
Whatever it is ? it's fascinating!

Greetings Izzard:

My own cat doesn't do this when I sneeze but then I have a very loud
sneeze, nothing like a hiss.  To be truthful, the volume of my sneeze
would have probably been of assistance many years ago at the walls of
Jericho...

This from Amy Shojai at The Cat Site:

"Other letters this month included a cat's odd reaction to an owner's
sneeze, incessant meowing, and a cat upset about a new dog. My cat,
Seren(dipity), hates it when I sneeze but has never gone so far as to
attack me - she simply says "ack-ack-ack" in imitation of a lion
cough, scolding me for making such a rude sound. In fact, I believe
that to the cat, a human sneeze must sound very much like a feline
SPIT! If the cat thinks her human has SPIT! she may, indeed, take
offense and either scold (like Seren does) or challenge the verbal
threat in other ways."
From http://www.thecatsite.com/cat_behavior/behavior.php?a=march03_3

Makes perfect sense to me.  :)

Hissing and spitting is briefly explained at
http://www.sniksnak.com/cathealth/whydo.html and
http://www.craftycat.co.uk/behaviour/ref/hiss.html is written that
cats mimic hissing and spitting because it resembles that of a snake
which many mammals fear.  If you cats think you are "spitting and
hissing" when you sneeze, perhaps they begin acking because they
believe you are hunting prey and, therefore, they will assist with a
chatter.

At http://www.thedailycat.com/happytails/mind/mind_qa/archive/qa_242.html
it is noted that "A cat makes this sound [chatter/ack] when he's very
excited by the sight of prey but knows he doesn't have access to it.
This is his way of relieving his frustration."  Perhaps your cats
think that you have prey and they can't have access.

This about cat chattering (acking) from Cat Talk at
http://www.i-love-cats.com/meow/bigsocks/
"Cats will chatter when they are not in pursuit of a prey. Cats have
been known to chatter back at their human when they are caught off
guard or disagree with the human's tone of voice toward them. If your
cat gets in trouble, sometimes, he will chatter back at you, as if in
defense of themselves, saying, "Whatever it is, I didn't do it!"
Chattering, therefore, can be used in offensive or defensive
situations."

You may enjoy reviewing the articles "Cat Communication" at
http://www.petpeoplesplace.com/Care/Cats/004/27.htm and "Cat Chat: Can
Cats Talk?" at http://www.petpeoplesplace.com/Care/Cats/004/26.htm

A discussion among people about this cat behavior is located at
http://www.waferbaby.com/archives/2003/02/28/snotswamp/ but with no
explanation why.


Should you require clarification of any of the links or information I
have provided, please request it and I will be happy to respond.

Best regards (and gesundheit),
journalist-ga


SEARCH STRATEGY:

human sneeze cats
cat behavior sneeze
cat meow sneeze
cat miaow sneeze
cat meow "I sneeze" ack
"big cats" hiss spit
"cat communication"
cat sound "ack ack"
why do cats hiss chatter

Great stuff, Journalist; nicely strung together!  The suggestion of
being excited/frustrated about prey which is inaccessible is
interesting (as I said, he does this when there is a fly buzzing out
of reach).  I'll have to watch them more closely next time with your
research in mind!  Now, that was way more than a $3.50 answer, so have
a tip :)

Maybe you are all allergic to cats and when you sneeze they're
laughing at you for not recognizing it. Cats are like that you know.
Very sneaky creatures indeed.

tutuzdad-ga

Thank you, Izzard, for the rating, comments and added generosity! 
I'll be paying attention, too, should anyone with a polite sneeze ever
sneeze in front of my cat. lol

Best regards, and wishing you safe and joyous holidays,
journalist-ga

I raise Maine Coons and agree that they are the most awesome of all cats...

I think that your cat is very well mannered and is simply saying the
kitty equivelent of "Bless You" !!

LOL

Thanks so much for the insight to this mystery! I have a cat that has
been doing this for years and I could never figure out why! It never
occurred to me to go looking for an answer...