If the hashrate drops, then the system automatically reduces difficulty which will in turn mining more lucrative. That way, the hashrate will never drop doen to dangerous levels, but instead will regulate itself so that approx. 6 blocks per hour can be processed.
I have to diagree. Bitcoin is far from being safe from 51% attacks.
It all depends on demand - people willing to pay for bitcoins. If there are "to few" of them, the price will drop (simple market law).
Low price means people are leaving mining (and some of them bitcoin) and - yes - difficulty will drop, you will get more bitcoins and your total $ earnings may stay the same ...
people leaving the coin means less "players" and therefor less demand (less customers, less shops, less everyone). This CAN become a downward spiral of the price.
Don't fool yourself - if you look at the top miners in the pools you will see that, lets say, 100 GHash are already a danger zone for 51%-attacks. Of course bitcoins has 10 times the amount right now - but it had much more at one point.
Please remember - this is a WHAT IF, a WHAT COULD HAPPEN scenario. I hope this never to happen - but look at the alt-chains.
They are "what if" scenarios becoming reality.