I was thinking the same thing. I read that some groups were concerned that hackers or saboteurs could hack into a nuke power plant and make it malfunction. Surely, no one would connect the control systems for a powerplant to the internet? Hacking into secret information is a little more believable. Maybe they got in through some obscure connection to a nonsecure network and found a bridge to a secure network? Or maybe they had an insider?
I bet almost all of this is insiders. Look at what a peon like Manning can get access to. The foreign workers have full access and can easily copy over off network to send home.
The insider (multiple, probably) seems like the likely scenario. This sort of material is *supposed* to be air-gapped, so someone either moved files to a vulnerable location or simply did the Bradley Manning USB drive thing.
"Hacking" is a handy explanation for putting the blame on a faceless national rival and potential enemy, rather than admitting that the DOD has been heavily infiltrated.