Google tries to find the keywords that are suddenly popular in recent news articles, blog posts, Twitter messages. The most popular 10 keywords are available at Google Trends and they trigger a real-time search OneBox.
"When they are relevant, we'll rank these latest results to show the freshest information right on the search results page. Try searching for your favorite TV show, sporting event or the latest development on a recent government bill. Whether it's an eyewitness tweet, a breaking news story or a fresh blog post, you can find it on Google right after it's published on the web," explains Google.
Here's an example of real-time search results page for [Google]:
Surprisingly, Google sorts the results by date, so you won't be able to use the service to find popular tweets, news articles that are frequently referenced, you'll still find spam messages and re-tweets. There are plenty of search engines that aggregate messages from micro-blogging services (Twingly, FriendFeed), but none of them managed to rank the results by relevance.
In May, Marissa Mayer discussed the significance of Twitter for Google:
"What's really happening in Twitter is that there are a lot of clues in it in terms of what's happening that's interesting overall. It's similar to what we see in Google Trends, where people will often type what they're interested in into the search box, and we can make some predictions off of that. So we are interested in being able to offer, for example, micro-blogging and micro-messaging in our search. Particularly in Blog Search and possibly in Web Search."