There is a PPA repository for adding Gnome 3.8 to Ubuntu. See these:
However, be advised these PPA repositories are unsupported by Ubuntu. Also, you need to understand that adding the cutting edge Gnome 3 desktop to Ubuntu 12.04 LTS essentially defeats the purpose of running the long term support version of Ubuntu.
The idea behind the LTS versions is that they run stable, tested, and trusted packages for increased stability and reliabilty. This is a tradeoff that you make between long term reliability and cutting edge features.
Adding Gnome 3 to Ubuntu with Unity should give you the option to login to Gnome or Unity when you boot the computer. However, I am not 100% sure of this.
There is a new Ubuntu Gnome remix available also. Unfortunately, the Ubuntu Gnome remix for the upcoming Ubuntu 13.04 will still be using Gnome 3.6:
However, you could easily add that Gnome 3.8 PPA to the Ubuntu Gnome edition 13.04 if you wanted to run the latest Gnome stuff. I would think that this would be the best way to go if you really want to be running Gnome 3.8 on Ubuntu.