ambience is the theme gtk which comes by default in Ubuntu since version 10.04 and since then it has been receiving slight modifications with each release.
Andrea Cimitán has shared in Google+ a screenshot of another change that will include in oneric this beautiful theme, which will now have a black toolbar.
If you compare it with the current version, you can see the elegance that the new settings bring.
Very good to the design team at Ubuntu. Now (from my humble opinion) they would only have to work a little more on the set of icons to fit them more with the black tones.