Yes it would take only about 6 days worth of BTC mining being sold to completely empty the BitStamp exchanges of USD (Gox because of lower price and higher USD would take 1 month).

People need to understand that BTC 'market-cap' of BILLIONS is an illusion created by hoarding 99% of the supply and keeping a mere ~20K (0.1% of Total) coins on exchanges to be offered in exchange for USD.  This gives us an absurdly shallow and volatile market, you hardly even need to be a 'whale' anymore to move the price.

A 'safe' valuation for a BTC is the 'total-supply' that everyone loves to talk about divided by total offered dollars, which would give us a value of around $3.50 right now if you total all the USD on all exchanges, round up to $5 even for all the Euro and Yuan markets too.  That's a valuation that people could feel confident in not losing value at due to this kind of 'bank-run'.