One of the best shields for conspiracy is the notion that entertaining conspiracy makes you a nut. It is important to debate the depth to which conspiracy pervades our society, however, there are sufficient cases of historic fact that where wars, elections, and assassinations have been the result of those that have earned legitimate power, exercising it through nefarious means.
It would be a fairly simple matter for a wealthy governmental or non-governmental agency to funnel cash to the accuser of DSK. However, the veracity of the law enforcement before his resignation, and the timing of these revelations suggests to me an influence upon the legal system.
Do I think that DSK was set up by a US agency? I don't know. However, I wouldn't put too much of my own money against that possibility.