The Google Caffeine update is already live in one data center. This is basically rewriting of some basic architecture and there would not be any visible UI changes.
Google Caffeine is one update which Google has been working on and asking end-users to test for a long time. Google Caffeine also opened up a preview on these search results on its sandbox subdomain. This update would be on its architecture which would include indexing, crawling and serp changes. What is strange is that, this time Google requested webmasters to test the search results and also provide feedback on it.
On testing the new Caffeine update on their sandbox, you would hardly notice any difference in search engine results however, image, news or blog result changes were identifiable.