You can install Unity alongside Gnome, and choose between them using the menu at the bottom of the login screen.
To do this, open Synaptic, go to Settings -> Repositories, and in the Other Software tab click Add and paste this line:
ppa:canonical-dx-team/une
Save your changes, then click the Reload button in Synaptic. When the new package lists are loaded, install the package called Unity. When you log out, you should be able to choose "Ubuntu Unity Netbook Edition" from the menu on the login screen.
To do this, open Synaptic, go to Settings -> Repositories, and in the Other Software tab click Add and paste this line:
Save your changes, then click the Reload button in Synaptic. When the new package lists are loaded, install the package called Unity. When you log out, you should be able to choose "Ubuntu Unity Netbook Edition" from the menu on the login screen.