The report is still unofficial, but according to the gang at GTChannel, the kit will indeed be sold in the U.S. in about six months. The 370Z was just released for official sale in the U.S. yesterday but it has been on sale in Japan for most of December.
For the 350Z, Nissan never released a specific S-Tune package in the U.S., though it did offer S-Tune and R-Tune parts and graphics through its Nismo catalog and dealerships. The Nismo 350Z that eventually came along in 2007 saw a number of the JDM modifications that went into the S-Tune 350Z also used on it, though between seam welding and construction at Autech, the Nismo 350Z was also a unique car.
It's not yet clear if the S-Tune kit for the 370Z - available in both 'Aero' and 'Performance' packages - will be available pre-packaged or if they will again be special-order pieces that can be put together to 'build your own'. If the report holds true, however, we only have to wait six months to find out.