I agree that the key is whether the employee’s “primary duty” is the performance of clearly exempt work. The current “white collar” exemption regulations (effective since 8-23-04) explain the requirements for the various exemptions that result from FLSA Sections 13(a)(1) and 13(a)(17), and there is a separate section dealing with “primary duty.” These regulations (29 CFR Part 541) are accessible at http://www.wagehour.dol.gov (look for “Fair Pay” and click on “regulations”).
In those regulations DOL makes it clear that the agency does not hold an hourly payment for extra work (whether or not the duties are exempt in nature) to be in conflict with the salary basis test. Perhaps there is case law that has held otherwise, as Kevin is not the first attorney who has recommended avoiding hourly payment. I can assure you, however, that DOL (the enforcement agency) will not raise the method of payment for the extra work as an issue (if an investigator does do that, he or she is simply not well trained and will not be supported by Wage and Hour Division management).
Whether the Saturday work in a nonexempt job will compromise the validity of exempt status is dependant on how solid the exemption classification is in the first place. If it is shaky as to whether the employee’s primary duty is exempt work, then the Saturday hours (if this is repetitive or frequent) might tip the scales toward non-exemption. DOL deals with a similar question in an opinion letter, and, from my recollection of the letter, I believe that my response conforms to the (more detailed) information available there. In summary, I suggest that you examine the strength of the current exemption determination, and if there is no doubt and there is obviously a great deal of “wiggle room” (i.e., much nonexempt work could be done without causing loss of exemption), the Saturday job will not be a problem.