You may have heard speculation that the NSA has secretly broken the strong cryptographic systems used to keep data secret — after all, why collect all that scrambled data if they can't unscramble it? But Bruce Schneier argues (convincingly) that this is so impossible as to be fanciful. So why have they done this? My guess is that they're counting on flaws being revealed in the cryptographic implementations in the field (or maybe they've discovered such flaws and are keeping them secret). Or they're hoping for a big breakthrough in the future (quantum computing, anyone?).
Right now the upper practical limit on brute force is somewhere under 80 bits. However, using that as a guide gives us some indication as to how good an attack has to be to break any of the modern algorithms. These days, encryption algorithms have, at a minimum, 128-bit keys. That means any NSA cryptoanalytic breakthrough has to reduce the effective key length by at least 48 bits in order to be practical.
There's more, though. That DES attack requires an impractical 70 terabytes of known plaintext encrypted with the key we're trying to break. Other mathematical attacks require similar amounts of data. In order to be effective in decrypting actual operational traffic, the NSA needs an attack that can be executed with the known plaintext in a common MS-Word header: much, much less.
So while the NSA certainly has symmetric cryptanalysis capabilities that we in the academic world do not, converting that into practical attacks on the sorts of data it is likely to encounter seems so impossible as to be fanciful.