Google tries to help with those sorts of search sessions. 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.
For example, I just did the search [iphone] and then the search [wikipedia]. In addition to the regular results for Wikipedia, Google also surfaces the page http://en.wikipedia.org/wiki/IPhone . A good way to see that Google is doing this is to look for a phrase like "You recently searched for iphone" under the newly-surfaced results. Go ahead and try it with the search [twilight] and then the search [wikipedia] for example.
Between Gabriel's article and the WSJ article, words that are reported to provide this behavior include iphone, nexus, obama (but not romney, because there wasn't enough information for this word at the time the data was generated), tablet, twilight, computer, health, speech, iraq, sports, social security, and stock.
Just to reiterate, this algorithm affects 0.3% of searches on Google. Most Hacker News readers are savvy enough to search for [iphone wikipedia] instead of breaking that search into multiple queries. However, if you don't want Google to surface additional results that might help with your current query, Google has a support page telling how to turn off search history personalization: https://support.google.com/accounts/bin/answer.py?hl=en&...
As an aside, I wrote a blog post ~4 years ago to preemptively debunk the idea that Google skews our search results for political reasons: http://www.mattcutts.com/blog/google-search-and-politics/ We simply don't do that.