There's a video from the RSA conference in 2011 with Dickie George, who was the director for Information Assurance at NSA when DES was being reviewed. He claims that the agreement between NIST and NSA was that: 1, NSA would only change things if they could find a specific problem with the cipher, and 2, NSA promised that DES would have security equal to its key size. The implication is then that they decided that 56 bits was how secure it was, and then picked that as the key size.
You can believe him or not, but I don't see any particular reason not to.
Thanks for the video. I ended up watching the whole thing. My interpretation is a little different than yours: I think George is saying that there's no point in having a key longer that 56 bits given that the goal is 56 bits of security, but he's vague about where the requirement for 56 bits of security came from. In any case, the video certainly supports my larger point that the idea that NSA would sabotage a crypto standard was mainstream within crypto circles, even in the '70s.
You can believe him or not, but I don't see any particular reason not to.
link to the video, the relevant bit is ~8min in: http://www.youtube.com/watch?v=0NlZpyk3PKI