Wikileaks has servers in dozens of countries, so even attempting to shut down their server would be a waste of time, and would only play into Wikileaks' hands, solidifying their "underdog" or "David versus Goliath" position.
The NSA likely isn't even wasting their considerable supercomputing power on this, since they and everyone else knows the documents will eventually be released, anyway. The posting of an encrypted file named "insurance" is all part of the childish game of someone sticking their tongue out at the US. They'll play that one for all it's worth, and then when publicity is high enough (or begins to drop off, so as to boost it again), or when they get enough donations from people who want to support them, they'll simply release the documents that are contained within the encrypted file. If Wiki was really and truly all about what they say they are all about, they'd have already released all of he documents, rather than trying to squeeze publicity out of it.
They aren't at all likely to release the password, as that would give the NSA and others an uber-huge advantage to use with future encryption issues, since knowing the password will tell the NSA the manner in which it was generated, which could cut the time required to break future encryptions down to hours rather than 3 times the age of the universe. If the password were to be released, then it's a safe bet that the same encryption method would never again be used by Wikileaks.
The owner of Wikileaks is Australian, not a US citizen, and has no fixed address, so none of the US laws, or threats, apply to him in any way. He knows it, and so does the US government.