Non Disclosure Agreements/NDA's are somewhat common nowadays depending up the nature of an individuals business operation and I have no problem with that. The only exceptions would be if I were required by law to report my knowledge of said information to appropriate law enforcement agencies or if I were complled to testify in a legal proceeding, even then I would first seek legal counsel to make sure it would be appropriate to do so.
However if the information/data etc.. is related to criminal/illegal activity that the client is engaged in then the NDA no longer applies.
Basically, if you are doing everything legal and above board you have no worries as I will honor the NDA (if one is in place) and if no NDA is in place I will still keep everything confidential.