I think that people here are missing the point. While they can't eliminate bitcoins themselves because of the peer-to-peer network and blockchain/wallet system, if the government (for the sake of argument let's say the US govt.) wants to stop the spread of Bitcoin they can "ban" them, and then prosecute any businesses that accept bitcoins and freeze their USD funds (like WordPress).
Then they go after Dwolla, BitInstant, etc..
They don't have to wipe Bitcoin out of existence, but if they are able to stop what is currently some decent momentum towards wider acceptance/adoption by forcing large scale legitimate businesses out of the market, then the value of BTC will drop, demand will fall, and it will go back to being something that hackers play with on Tor.
I'm not saying that this is about to happen, or that they have a huge interest in stamping out Bitcoin right now, but it is very naive to think that they can't do it. If they provide a strong enough financial disincentive for merchants to accept bitcoins, they can strangle the movement in its cradle, so to speak.
True, only a few key services would have to be disabled to make paying with BTC much more harder.
Say the US govt. would make up some money laundering excuses and shutdown exchanges or just disable payment for them (Mt.Gox, BitInstant etc).
Suddenly the most important nodes and all bootstrap nodes would disappear from the network (DDOS).
Mining pools wouldn't work anymore because the domains will be seized (seems like all important ones have US domains).
Users couldn't communicate anymore efficiently since the biggest forum would be shutdown.
Seems scary...