From the casual attention I pay to this, I thought Coinbase was one of the, if not the, largest US exchange at this point. I'm surprised no one has even mentioned it here. Am I missing something?
From Brian Armstrong, Coinbase Agent, via the coinbase support site, 11 Nov 2012:
"We integrate with a variety of exchanges and also hold our own reserves. When you sell your bitcoins for USD we usually sell them to other Coinbase users at some point down the line (this is where our reserves come from), but we may cash them out if needed. In general, our goal is to abstract away some of the complexity so it's a simpler process for our users. What we have now is a good start but not finished by any means. Hope it helps!"
"Coinbase buy price is pegged to Bitstamp" (Dec 2013):