The Nikon staff previously answered with regards to using the flash when recording movies:
"If you're working in less than adequate light while creating a Motion Snapshot or using Smart Photo Selector, attach the Speedlight and keep everything exposed properly. The SB-N5 Speedlight steps up to serve as a continuous light source for up to six seconds.
My first question is: Will there be an update released that allows the SB-N5 to serve during the 5 seconds Slow Motion (movie) option, as this function seems most sensitive to low light.
My second question is: Where (at the Net) would it be natural to submit such an feature request ?