The US claim jurisdiction on anything with a .com - and they've seized control of .com domains hosted abroad previously.
> No single government can take it down.
The US would find it trivially easy to take down Bitcoin if they wanted to.
"The US would find it trivially easy to take down Bitcoin if they wanted to."
No they cannot. For the same reason that the US cannot take the illegal drug market down, or cannot take Bittorrent down. Decentralized systems or industries (whether in the digital or real world) are very resilient.
Care to elaborate?
The attacker generates a blockchain that's larger than the real blockchain. That prevents any transactions being confirmed. To do this the attacker needs as much computing power as the rest of the Bitcoin economy, and they need to want to spend money to destroy Bitcoin. (Because they'd make more money either mining or double spending).
Even if that did happen, I think bitcoin could recover. All you'd need to do is get everyone to agree on a point in time where the blockchain was untarnished and to reset to then. That would be easier than it sounds, given the small number of existing clients. It would be damaging, but only in the short term.
The US has NO jurisdiction on many of the Bitcoin exchanges (btcchina.com, btc-e.com, bitcoin.de, virwox.com, cavirtex.com, bitcoin-central.net, etc). In fact, if the US were to seize the biggest US-based exchanges (mtgox.com, etc) I would expect the exchange rate to naturally go up on all the other worldwide exchanges, because of the reduced supply of Bitcoins due to the seizure. (The US only represents a fraction of the total Bitcoin economy, so I doubt non-US Bitcoin users would sell in panick.)
That is a nice consequence of the fully decentralized nature of Bitcoin. No single government can take it down.