It's not easy to buy $500,000 worth of bitcoins on the open market at "current value".
Someone bought ~$1mm on gox a few days ago without too much trouble I think
They did, but it sparked a frenzy of buying shortly afterwards...
Cost of conversion would be a potential reason.
Right, if there are supposedly buyers from abroad who want to buy, we can save money in currency conversions and transfers across borders.
Doesn't even have to be "from abroad" - if a buyer has a stack of bitcoins and I want a stack of bitcoins, going through an exchange is going to be expensive given the pretty broad spreads (if there's even the necessary liquidity).