It isn't as easy as it looks, particularly for an implosion plutonium bomb like Fat Man. I don't know how difficult a gun bomb like Little Boy would be to make, although it would have to be easier than an implosion bomb. There is an entire engineering science devoted just to machining plutonium and uranium, both of which I believe are pyrophoric.

I agree with you: either the bad guys have the complete blueprints already, or they have access to the same Encyclopedia Brittannica plans everyone else has. Not a major issue.

More of a problem are plans for more conventional weapons available on the Net, like napalm and other make-it-in-your-garage nasties that appeal to the Columbine types.