So you should be able to use this in Gnome desktop in Natty Narwhal as well. Anyhow, the main difference in this theme comparing with the original Ambiance (heck, there's even a Gnome3 theme as well!) is the lack of borders which makes it looks awesome btw :).
Other than that, it also has darkened tool-bars, better looking buttons, smooth transition between tool bars and title bars, enhanced/new sliders are among few of its main features.
Enough bragging Gayan!. Where can I get it?
Parley... :). Okay, um, first download the zip package from here. Then extract the content to ./themes folder in your Home.
If you don't have Nautilus elementary installed, then it is advised to edit the "gtk-2.0" file in this Ambiance theme (use your text editor) and delete the line which says " include "nautilus-elementary.rc" " which can be found at the end of the file.
If you want more information, then I recommend visiting simplegreat, the artist who created this awesome theme.