Specifically they are able to "beat the speed of light" between you and a website such that if you communicate with Bob, they can sent competing packets that will arrive before Bob's packets. They have realtime deep packet insertion able to inject arbitrary data into an TCP streams and can change file downloads **on the fly**. This can be done remotely.
Sourceforge do not have https downloads, so this is yet another reason to move downloads to somewhere that does - like github.
The NSA has the ability, right now to change every download of bitcoin-qt, on the fly and the only cure is encryption.
Revealed as part of the presentation is the fact that if the NSA has access to these capabilities, then so do others and in fact one of the things revealed yesterday was independently discovered already and published.
Same goes for the
bitcoin.org site - why are we dragging our feet on installing an SSL certificate and redirecting all http to https? While no solution is perfect, it's a lot better than zero defense.
You can see the irony of disseminating the bitcoin crypto-currency client in the clear.
For anyone who has not seen the video. You will be shocked by what is actually in the wild being used today. It goes way beyond anything imaginable even in science fiction.
Drak