I think you are confused. How does knowing a single address let you know how many coins someone has?
For example say you were involved with someone who provided this (just one I grabbed randomly from blockchain.info) as his address.
http://blockchain.info/address/1BJQDc7c9k9GGiBfoqZmnigGvEuQZCsVGPHow many Bitcoins does this person have?
My question is not really about the
total bitcoin holdings of a particular user, but more about the balance of a particular address, and the flow of bitcoins in/out of this address. As I wrote earlier:
anonymity is lost forever for these particular bitcoins.