a few things to note about circulation first
Hoarding is not what bitcoin needs right now. We need to spend and get retailers to think that they are missing out on business by not accepting bitcoin. That's when the masses will all start buying bitcoin driving the price to atmospheric levels. At that point hoard all you want.
when a coin is in the exchange it is locked in a third party service. thus it is not being spent on a retailer website. to me this is the same as me storing my FIAT in a bank and the banks are playing the Forex markets. meaning my money is not in circulation (being spend in retail)
i know that this is not entirely the case as "we" are the banks, so yes it is in circulation whether in an exchange or in our personal wallets to be used. so dont count wallet stored or exchanged stored funds as in or out of circulation.
the true way of knowing what is in circulation and what is hoarded is to look at the 12million coins. and compare it to the total coins 'transacted' over a period.
EG
http://blockchain.info/charts/estimated-transaction-volume820,000BTC transacted today compared to an average 300,000 a day for the last few months.
so in actual fact less coins have been hoarded now then ever before.
what you will notice though when it comes to exchanges is that for instance last year MTGOX was storing upto 150k of bitcoin at any one time.
but today only 12,500 coins.(12561.4 coins at the exact second of doing screenshot)
this proves that when supply ON THE EXCHANGE is low the prices go up.. even though there are more coins being transacted today instead of cold stored (hoarded in personal wallets). meaning that retail or peer-2-peer trading is increasing and there is more circulation. exchanges are seeing a decrease in circulation.
Thus you have to ensure you understand the difference.
now back to the topic title
. knowing that if a rich guy wanted to buy up all 12564 coins on MTGOX it would cost 77,662,796,314,500 for that very last satoshi, then he would have to do the same at bitstamp, vircurex, btc-e and then try calling out to the public for their cold stores, for him to EVER have the chance of ever owning "all the coins"
so i dont think anyone will ever own all the coins