Are the 160 bit hash values generated
by SHA-1 large enough to ensure the
fingerprint of every block is unique?
Assuming random hash values with a
uniform distribution, a collection of
n different data blocks and a hash
function that generates b bits, the
probability p that there will be one
or more collisions is bounded by the
number of pairs of blocks multiplied
by the probability that a given pair
will collide.
(source : http://bitcache.org/faq/hash-collision-probabilities)