I would consider the quantity of coins in circulation be the sum of the balances in all addresses that have been either on the sending or receiving end of a transaction in the last 365 days.
Or maybe better the total of all transaction outputs from the last 365 days which aren't yet redeemed.
I could have a million BTC in an address that hasn't been touched for 2 years, and then add 10 BTC to it today. That shouldn't add a million and ten to the "in circulation" number; it should just add 10 BTC.
My assumption is that if you have an address that is accumulating coins, the address is still hot - the million could come out of it at any time, otherwise you wouldn't allow about $50 of value to end up in it.
I am looking to remove from the quantity of bitcoins all coins that are either lost, or tucked so far away that they shouldn't even be counted. If you take that figure, and multiply by the current exchange rate, you can get a rough true value of the bitcoin economy.