Getting back to the topic at hand, I do think that the circulating supply should be used when establishing a Market Cap as that is truly what matters. If somehow some of those estimated 2 million Bitcoins assumed lost were suddenly dumped on the market, the price would crash like nothing we have seen so far. Just look at all the hype and blame for the Mt Gox bankruptcy trustee dumping a few of the 200,000 coins onto the market, so image the impact of 10x that.
Another negative to the whole basic "marktetcap is based on the total supply" argument is that every time the market does go up or down, headlines scream "Bitcoin Loses 10 Billion in One Day" due to a $500 price decline, when in actuality the relatively small number of coins traded make the impact far less than that. Most coins are locked up in one form or another, whether that be they are lost, or just being held in storage for the long game, and the few that are traded day-to-day probably only represent a very small percentage of the total supply.
These are some good points. Circulating supply is just all of the coins that could possibly be sold at any time, as there is no way of knowing for certain if coins are actually lost or irretrievable then they should be included in to circulating supply. I think that the market price is already somewhat reflective of the fact many of these coins are lost as it's a fairly common exception for example that Satoshi's wallets will never be touched.