The problem with all these conspiracy theories (and the same applies to many gold-bug theories) is that they fail to explain why would any asset/currency/commodity/whatever be a "threat" to US dollar and consequently why would the US government have an incentive to kill it. In case you haven't noticed, the US issues the US dollar, which is a currency backed by its monopoly in taxation in the USA. It doesn't care if it loses value, in fact it has an interest to see it go down in value. It has an interest to see people using it to do whatever (even buy gold, bitcoins, or oil futures) so that they can tax the activity generated from it. Bitcoin in this sense is NOT a threat to US govt interests, it's more of a blessing. A reason for more economic activity (more hardware sold, exchange fees paid etc.) and therefore more taxes paid.

You can make an argument for being a threat to the banks, but not to the government. Even that argument wouldn't fly though, because if and when bitcoin goes mainstream, existing banks and mega-corporations will be in a very good position to actually offer services such as exchanges, online wallets, etc. that could threaten existing businesses in these areas, and therefore replace the current revenue stream from banking operations with equivalent stream from bitcoin equivalent operations.