Even Darkcoin, Monero, Boolberry and the like make some type of digital footprint on the Internet that is trackable.
In my mind, l33t hackers will always be one small step ahead of the NSA and the NSA will always be miles ahead of the general populace.
I don't know enough about the governments abilities to know what the NSA can do and I don't think anyone else here does either. I know it's possible to parse the blockchain and get trails from it that most people wouldn't think possible.
I have a simple rule you can follow that makes this problem moot. Don't do anything on-line or buy anything on-line that you wouldn't show your mother and grandmother in church on Sunday Christmas morning. If you must buy drugs or some other illegal item please only use fully anonymous U.S. Dollars. For further security I suggest only purchasing items on a university campus where all items are readily available.
if only i knew this back in '98
The first class I took in college was a no credit one taught in a dorm room.