Well...
Someone please correct me if I'm wrong, but I think the value of BTC (and therefore, how people generate it's market capitilization), is based solely on what someone will pay for it, or sell it. The bid/ask spread and depth, if you will.
So, what I'm saying -- if someone very rich tried to buy "one Billion Dollars" (with their pinky finger!) of BTC in one day (or 1 hour), the price of BTC wouldn't "double" to $200, and then have a $2B market cap. It would be worth, well, maybe 1 million USD, perhaps? Total guess, but it depends on the "depth" and prices then available on the ask side.
If so, that would mean ($1,000,000 USD * 10,966,000 BTC) a market cap of... Eleven Trillion USD. Or at least it would be for a few minutes.

I'm mostly posting this because I need more posts to get out of Newby land.

But thought it might spark more discussion...
Maybe we should consider putting some six figure ask's up at MtGox, hmm? Couldn't hurt, I suppose.