Since 2008, Google has been using previous query to refine and personalize the search results a user may see. It is nothing new and a smart and detailed searcher might take notice when it happens.
When does it happen? When you search for something and then go back a few seconds later and search for something else, Google may decide to combine the searches and give you a search results page that uses two queries instead of one. For example, if you search for [iphone] and then decide to search for [wikipedia], Google may decide on the second search to give you search results for the query [iphone wikipedia] instead of just [wikipedia].
What is new is that Google's Matt Cutts is defending Google on the topic and let a figure on how often does this come to light.
In a HackerNews thread, Matt Cutts from Google said Google does this for about 0.3% of queries. He wrote:
For 0.3% of queries, if we see a search for a query A and then another search for query B, and there appear to be good results related to both A and B, then we may surface those results.
So there you have it, previous query seems to be used in personalized search about 0.3% of the time.
Forum discussion at HackerNews.