We recently did work for a very large corporation that switched from net 60 to net 180. I'm not sure if they did it for cash flow reasons, but I doubt it. We're a small company, so it was tough for us considering we'd be paying developers 6-months of salary before we'd see a dime. They did conveniently offer financing so that we could get paid sooner, albeit with interest.
It seemed to me like large corporations feel that they can strong-arm their vendors into unsavory terms because of their clout.