This is true, of course (and a pet peeve of mine when people naively calculate e.g. how much it would cost to replace all coal-fired power plants in the world with solar). However, these days MtGox's average daily volume is in the $1m range. I think you could sell $1m BTC over a couple of weeks without anyone even noticing.

Same applies to a lot of wealthy people in shares. If they sold all their shares, they'd make the share price plummet and they wouldn't get anywhere as much as you initially quoted.

As volume picks up (market depth has nearly tripled in 2 months) this is a lesser problem.