The Federal Reserve will probably be the first ones in line to snatch these up with a big batch of freshly printed dollars.
Provided they really manged to get their greedy fingers on them, i think thats something the foundation should get involved with, because it would be catastrophic to have the feds holding and dropping such a large amount on the market. Afterall its just freshly printed bucks, so they dont really care.
I'd imagine that like other seized items, they would auction them off, even though presumably selling them off slowly would maximize their value. Then it would be up to whoever bought them to decide what to do with them. Someone who got them for a song would have a strong incentive to start out with a round of profit-taking and cash in some of them, regardless of the short term impact to the Bitcoin price, stopping when it became less profitable to do so.
Of course, other interests (say the Winklevoss twins) could also pick them up in such a transaction, or someone else with enough money to afford picking up a large quantity of a highly volatile asset. If the Winklevoss twins got it, they'd have a good incentive to hold onto it and put it in their fund, or something similar, and it wouldn't have much impact on the exchange rate because it would still, essentially, be tied up like it is now, just in a venture rather than with the government.