NASA recently announced that the EmDrive, which uses microwaves to create thrust, is actually real. While primarily aimed at space this could also be used here on earth, for a hoverboard.
Finally the hoverboard dream from Back to the Future II could become a reality. It was presumed the hoverboard would be powered by some sort of magnetic device but it could end up being the EmDrive.
Despite NASA initially rejecting the relativity-based drives as impossible, according to the law of conservation of momentum, it's now saying microwave thrusters appear to work.
The larger Chinese version of the EmDrive was able to produce 720mN – about enough to lift a few ounces of weight. In frictionless space that's enough to propel a ship slowly and steadily up to super speeds. Here on earth more power will be needed to hold bodyweight on a hoverboard. This could be achieved by creating a superconducting version of the drive.
While the hoverdrive would be able to hold the rider on the board it would still require forward motion, from pushing off like on a normal board. That said if the drive could be made efficient enough there might be enough thrust for forward drive too. As Back to the Future's Biff told us we'll need that for riding on water where "you need pooower".
So the next question is when? Not for quite some time still. We'd expect hover cars and bikes before its small enough for a board. Imagine Formula 1, hovering, or the X Games on hoverboards and hoverskates.
This will be technology that filters down from the drives NASA develops. But it's finally possible within our lifetimes. Let's hope skateboarding is still cool in the not too distant future.