Cryptographic hash uniquness (was [Haskell-cafe] Simple network client)

[Peter Verswyvelen]

>winds up having a write cache, which is mutable in practice.  The
>interesting thing is that the block's location is the cryptographic
>hash of its contents, which leads to all sorts of neat properties (as
>well as requiring immutability).

That's interesting.  When I developed a version control system for a customer, I also used a cryptographic hash as the database key of file+content in question, but I was afraid I might have clashes (two files with different content generating the same hash)... My intuition told me that the odds of two cryptographic hashes (on meaningful content) colliding was much less than the earth being destroyed by an asteroid... But this is just intuition... What does computer science tell us about this?

Thank you,
Peter