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'.
While I agree the market cap is misleading, your method also doesn't yield an useful figure for BTC value.
The amount of bids in the order book at any given instant is not an accurate representation of the total amount for which coins can be sold since the order book is very dynamic. It adjusts rapidly after every trade and event, and nobody should be so foolish to dump enough coins on any exchange to take it to $0 instantly (I don't count the mtGox hack of 2011, it has nothing to do with normal trading). In case of a massive bank run, many bids will be removed or placed at very low prices quickly.