From my experience, I was able to add the PPA, install and use GNOME 3, and then remove it effortlessly, without breaking my Unity setup.
I added the GNOME 3 PPA, installed all the required packages and libs, logged out and tried GNOME 3. Then I used this little neat utility called ppapurge (in the repos) to remove and revert all packages from the GNOME 3 PPA. It worked wonderfully. Just to reallly check it, I did a quick scan through Synaptic of all packages in the PPA, and none was installed.
It worked not only for me, but for others too. However, if you really praise your Unity setup, it's probably better not to touch anything. And anyway, the current GNOME3 experience in Ubuntu Natty is way below of what it should be, and I recommend you to try it using one of the live images made available on gnome3.org.