Normally, when we discuss Google updates, they are done without confirmation from Google. The search results on Google.com shift around, webmasters take notice and the update is named. Sometimes, Google confirms the major update and sometimes not. But never do we have Google create a sandbox (preview) area for us to test the update prior to it reaching the main Google.com results. At least, not until today.
You won't notice much of a visual difference, but you may notice ranking differences between the live results and this sandbox area. Here is what Google said about this update:
For the last several months, a large team of Googlers has been working on a secret project: a next-generation architecture for Google's web search. It's the first step in a process that will let us push the envelope on size, indexing speed, accuracy, comprehensiveness and other dimensions. The new infrastructure sits "under the hood" of Google's search engine, which means that most users won't notice a difference in search results. But web developers and power searchers might notice a few differences, so we're opening up a web developer preview to collect feedback.
Google wants your feedback on these results. To give it "do a search at http://www2.sandbox.google.com/ and look on the search results page for a link at the bottom of the page that says "Dissatisfied? Help us improve." Click on that link, type your feedback in the text box and then include the word caffeine somewhere in the text box."
Again, this makes history in that Google has never released a public preview of such an algorithm, index and infrastructure update to their search updates.