How do you get "1" by using three "9"s and only one mathematical sign?  (9 9 9 = 1)?

My best guess: 9^(9-9) = 1

I don't know exactly what you would count as a "mathematical sign," so
I can't be sure whether or not this is acceptable, but 9^(9-9) is the
same thing as 9^(0), and any number raised to the 0th power is 1.

Probably not the answer, but under some Boolean logic True = 1

So :  (99 > 9) = True = 1

Note that 9^(9-9) = 1 can be written with superscripts so that the
only "mathmatical signs" are the minus sign and the equals sign. If
you are counting the equals sign as a mathematical sign, I don't think
this puzzle has an answer.

.999 is approximately 1
or 
you could put a slash through the equal sign to say 999 (doesn't equal) 1

One interpretation of a mathematical sign would be the signum function.

According to this website, signum can be represented by one symbol.
http://members.aol.com/jeff570/functions.html

Therefore:
sgn 999 = 1
is a possible solution.

hi....   9+9+9 = 27     2+7 = 9 ???

I think that pinkfreud can claim the prize:

 (9-9)
9     = 1

Cathrene,

If you ever learned the definitive answer to this puzzle, I'd like to know.

~Pink

a few other possibilities:

    9
9/9  = 1
[this operates on the same premise as pinkfreud's response (that a
raised number is not counted as a mathematical sign)]

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|999|=1
[The cerdinal number (number of elements) in a set that contains only
the number 999]

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ceiling(.999) 
[A sligt modification of kerplode's first guess]

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     9
log 9   =1 
   9

[Log (base 9) of 9 to the 9th power.  this is without a doubt not the
right answer...but it may still technically qualify]

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All said there are two (three if i can include log) 'correct' answers
under the assumption that a superscript is allowed (as opposed to ^). 
That leads me to believe that they may be looking for a different,
unique solution.  Of the solutions I can see here I think sign(999)=1
is the only answer that uses only one operation.