I'm afraid to say that we have no idea about their exact release date. I myself contacted The V-Cube company directly and, the person that replied told me to sign up to their newsletter; when the new cubes are released, you will receive a newsletter telling you so.
By the way, there is also a V-Cube 6b coming out - that's basically a pillowed version of the V-Cube 6. Along with that, there is also a V-Cube 4a coming out.
The V-Cube 11 looks to be the best of them all... what do you think