Christian Poulsen has made his own "working" BB-8 droid, and has posted the instructions online. His working model of the droid gives us some hints at how BB-8 likely works, and it should be easy for anyone with a bit of experience to put together.
We know that the company behind Sphero is making an official toy, and this hacked version of the droid uses a Sphero toy modified with a strong magnet and a fabricated BB-8 head to work. The process of making the droid is rather simple, and looks great in action.
Here's the kicker, though: as you can see from the Star Wars Celebration video the "real" BB-8 has an articulated head that can be moved separately from the body. This can likely also be done with magnets, it would just have to be a magnet that's on an articulated arm.
Still, for a hack that took only one day, Paulsen's take on BB-8 is very impressive and captures the fun of the droid effectively. We'll see how the official version looks, but for now this is a pretty good weekend project.