I just completed an article for Nikon about using HSS, or high-speed sync, with speedlights to give you the edge when existing light is just not quite cutting it. HSS is amazing technology, but you have to understand it's limitations and how to best use it.
Follow this link to the article.
One of the lessons I learned in writing this article was to keep detailed model releases easily accessible for all your images! Fortunately, I use an ipad app to record my model releases, so most all my records were easily accessible. I did have one model in Puerto Rico that I had to track down to re-sign a release, but she was happy to do so.
Nikon is very particular about being legit with everything they post in their media (understandably so). They even needed me to get officially notarized releases from any images that included children in them! This is not the norm, but I can see how they want to cover their derrières. Fortunately, I was still able to get a hold of my past clients and convince them to make a trip to a notary public to sign the releases.