The most searched-for word in Cambridge Dictionaries Online in 2011 was the word 'dictionary'. CUP has published the top ten words searched for on its site last year. After dictionary, in order, came bear, eat your heart out, lead, concern, lie, issue, despite, appreciate and schedule. (See article here.)
People often search for words that are in the news that particular day - tsunami, meltdown, riot, eg. During the phone-hacking scandal there was an increase in the number of searches for "hack" but not as big an increase as in searches for "humble", after Rupert Murdoch faced a Commons Select Committee and said "this is the most humble day of my life". Number 3 in the 2011 Top Ten, eat your heart out, appeared in an episode of Glee, and it is thought that this is why the number of searches for this expression rocketed on May 11th.
See the article on CUP's website here.