I'd imagine that it wouldnt be overly difficult per se to carry out the 51% attack, i mean, everyone here is assuming they will just build a massive mining rig and take over the network, although if bitcoin was being targeted to be shut down i'm guessing it would be more than one government.

what would probably happen is all domain names linked to bitcoin, exchanges and mining pools would be turned off, as many servers as possible seized then every government that was aiming to shut it down turns on there own mining rigs and takes control of a lot more than 51%. If bitcoin was to threaten the monetary system then globally i'd be worried about a lot more than just a goverment pool harvesting 51% or more of the hashes.

Of course this would all happen after an official banning and some high profile arrests splashed all over the media to scare people into not doing it, under the pretence that bitcoin is used for buying drugs, building weapons (if they believed that dork who did that talk at the confrence in london i forget his name), and other such things that require money laundering.

Before they carried out a 51% attack they sure as hell would reduce the network hash rate a fair bit using more conventional methods (then probably use the impounded equipment to carry out the attack or fund it with money seized from accounts)

Sadly i can see this happening, the way i figure if your the first person to recieve an asci and start churning out bitcoin like theres no tomorrow; then if you want to make profit you're best dumping them all on the market before the next person does.

That said if asci's ever do ship, as has been stated, lots of people will have them also, so you wont be making as much as you'd have hoped.

personally i think its all too good to be true, but i've been wrong in the past. The idea that someones invented a machine that will mean you never have to work again is kind of a little far fetched for my liking, but the idea is a good one.

p.s Hi everyone this is my first post