That would be the same as an attacker buying all US dollars and burning them.  It's not an effective attack, because people only "sell" their dollars when they are getting something of equal or greater value anyway.

So, yes, it is possible that someone could buy up all or many Bitcoins and then "destroy" the private key so that nobody can ever use those bitcoins again, but it really isn't a problem because the attacker is the only one who will actually lose wealth!

The only scenario that would be at all likely would be if something like a stupid government didn't like bitcoins and thought it was a threat, so they would spend $30,000 or w/e to buy all bitcoins and "destroy" them.  That would be pretty stupid though, because another ecurrency just like bitcoin would pop right up!

I suppose if a government actually wanted to hold off these ecurrencies, they should buy lots of the currency but not tell anyone.  However, instead of "destroying" it or never spending it, they should just manipulate the market a lot while it's young (buy a ton at once, then sell way undervalue, etc).  Then, not realizing the currency was being manipulated, people would just lose faith in these types of ecurrency...

Sorry that got rambly, I'm thinking out loud here.

 (Hypnotically) If I run any two (or more) unique e-currency exchanger I can displace btc e-currency from it circulation.
 I seems, "truly" system must include mechanisms that protect self from any fraud activity.
 And what do you think?

2Developers and BTC-system designers: please, join to discussion

I don't think you understood my point.

It is a lot easier to arrest and lock up the prominent bitcoin users than to manipulate the market.  Like it was pointed out, a market is not just a single currency, it can be a lot of different types of currency and goods.  Trying to manipulate one end or the other would be difficult.  It is better to just get the people - fear of being arrested is much more effective than trying to shake up confidence in the currency.

Destroying a large amount of currency just causes deflation and makes the remaining currency that much more desirable - imagine if you had a pallet of gold bars in your garage and nobody else did.

Maybe you don't agree with my assessment of the situation but what you posted was far from constructive.

In response to your original post, I don't believe the situation you describe is a big risk.

You can't destroy them all, more will be generated constantly.  That 21 million number is not going to be reached in the foreseeable future.  As you know, the generation algorithm adjusts itself to slow the generation rate intentionally.

What the OP described is called "cornering the market".  When someone tries to buy all the world's supply of a scarce asset, the more they buy the higher the price goes.  At some point, it gets too expensive for them to buy any more.  It's great for the people who owned it beforehand because they get to sell it to the corner at crazy high prices.  As the price keeps going up and up, some people keep holding out for yet higher prices and refuse to sell.

The Hunt brothers famously bankrupted themselves trying to corner the silver market in 1979:
"Brothers Nelson Bunker Hunt and Herbert Hunt attempted to corner the world silver markets in the late 1970s and early 1980s, at one stage holding the rights to more than half of the world's deliverable silver.[1] During Hunt's accumulation of the precious metal silver prices rose from $11 an ounce in September 1979 to nearly $50 an ounce in January 1980.[2] Silver prices ultimately collapsed to below $11 an ounce two months later,[2] much of the fall on a single day now known as Silver Thursday, due to changes made to exchange rules regarding the purchase of commodities on margin.[3]"

http://en.wikipedia.org/wiki/Cornering_the_market

I would be more worried about these shadow interests developing a rival chain rather than them giving away free money for bitcoins... the second option makes bitcoin users richer, whereas the first one could invalidate the whole value of their coins.

Mechanical Turk 

Get people to install the client, generate some coin, send to a common address.

It seems like that until you think about how to actually implement this. If someone came to you and said "Here's 1.5 Million Dollars, I want you to corner the Bit Coin market", would you be able to snap your fingers and it be done?

No way, it would take planning, hardware purchase, hardware setup, someone to get the client up and running, need people to get the Internet connection up, funneling the coin to whoever it was that wants it all. It would time and every second that your shadow corp isn't running, that's another random person out there generating coin and raising the bar for the coin generator.

So by the time it's all said and done, you would be going back to that person to ask for another 1 million since the network got harder while you were getting setup.

People and Corporations try to "buy" the lottery all the time. Sometimes it works, but more times it fails because you just can't buy luck.

Maybe if that person had started with 1.5 million when this project first started, but now I think it's beyond a feasible and logistics point to game the system. It's like trying to shut down the Pirate Bay now