Unity 8 is finally available for testing on Ubuntu 13.10 Saucy Salamander. But it will not be fully integrated yet, until Ubuntu 14.10, because the Mir Display Server has been optimized to work on Unity 7.
Unity 8 allows the user to switch from phone mode to tablet. For now, it is just an alpha release, so it has many issues. For now, the purpose of Unity 8 is to allow the get used to using Mir, by choosing it from the LightDM login screen.
It has been added tot he default repositories on Ubuntu Saucy, so all you have to do is:
$ sudo apt-get update
$ sudo apt-get install unity8
To launch it, just type unity8 in the terminal.