There is not enough liquidity on all the exchanges to sink such a large amount. It will drive the price rock-bottom if done in one swoop.
Actually, if Bitcoin succeeds magnificently in the future, this sale could be very good, on several counts.
First, the NSA will divest itself of a currency that in the future could be immensely valuable, thus preventing them from being able to wreak havoc on Bitcoin after it gets more established, and more of a threat to fiat currencies that inflate year-on-year.
Second, driving the price down will encourage some holders of large stashes to sell while they can. This will distribute the currency more evenly across the market, so that at the point the currency becomes valuable again, a Bitcoin "middle class" will be created, rather than a few zillionaires and a lot of paupers.
Third, the above will help to stabilize Bitcoin's value, as there will be less incentive or ability for a few very rich individuals to game the system, purposely attempting to drive the price down to reap the rewards when it comes back up again.
I suspect that each time Bitcoin crashes, the wealth contained in the currency gets spread more evenly across the population that is using it, because of the number of Bitcoins sold in order to drive the price down. This is a feature, not a bug. At this stage, we want Bitcoin to gain broad acceptance, and that won't happen until it has a large userbase. The crashes should help that userbase to grow. Each time Bitcoin crashes, it crashes less hard than the last. That suggests to me that it is becoming increasingly stable.