Ubuntu 13.10 will be shipping Unity 7 by default atop XMir (and fallback support for X.Org), Ubuntu 14.04 is a Long-Term Support release so it too will play with Unity 7, and then finally about one year from now Canonical plans for Unity 8 by default on the Ubuntu Linux desktop with Ubuntu 14.10. However, early adopters can begin easily trying out the new code.
You no longer need to build Unity 8 from sources, use any PPA, or deal with any foreign steps. Very primitive Unity 8 packages have been uploaded into the universe repository of Ubuntu 13.10 "Saucy Salamander" for testing. The Unity8 package upload happened this morning per Launchpad.
When installing the unity8 package, "unity8" can be launched from the command-line and it will show Unity 8 in its phone and tablet modes. The new Unity shell is just being launched within a new window for the time being until full integration is ready. The Unity 8 mode for traditional desktops isn't also yet in a good state with Canonical developers being very focused on the mobile experience at this point.