zero factorial = 1?

Combination, nCr=n!/[(n-r)!r!]

When r=n,

By definition,
nCn = n!/0!n! =1/0!

By the meaning of combination,
we draw n items from n items. There is only one way.
So, nCn=1.

Thus, 1/0!=1
Hence, we define 0!=1.

very sounding and supporting reason for 0 ! = 1

In mathematics, the factorial of a non-negative integer n is the product of all positive integers less than or equal to n.

0! = 1

as an instance of the convention that the product of no numbers at all is 1. This fact for factorials is useful, because

* the recursive relation (n + 1)! = n! \times (n + 1) works for n = 0;
* this definition makes many identities in combinatorics valid for zero sizes.
o In particular, the number of combinations or permutations of an empty set is, clearly, 1.

You can go to the link below for more info on Empty Product in Maths:
http://en.wikipedia.org/wiki/Empty_product

0! = 1 and 1! = 1 are definations.