Is that correct?  AFAIK, only some merchants (like Overstock) opt to receive bitcoins directly.  Usually, when you buy "with bitcoin" through services like BitPay, they give dollars to the merchant and sell your coins at some exchange.  Didn't BitPay say that they used Bitstamp, specifically, some time ago?  Isn't Dell using such a service?

AFAIK the price on exchanges is the reference used for off-exchange deals, too.  Large amounts may be transacted somewhat above or below that price, depending on which side is more eager to make the deal, but not very far from it.  And, conversely, off-echange deals will "spill over" to the exchanges to some extent. (If you need 10'000 BTC and find someone eager to sell 12'000 at 10% below market, why not but them all and sell the extra 2'000 on exchanges?  Even if it will cause the market price to fall 5%, you will make a nice quick profit there.  Ditto if vice-versa.)