For search engines, the progression of language and associations of various words is part of the daily grind. Yahoo’s buzz index & Google’s zeitgeist frequently cover word combinations and phrases that 30 or 40 years ago would have held no meaning whatsoever. This rapid expansion is on an ever upward-arcing curve as the Internet and communications make the formation of language more popular and accessible.
As SEOs, we need to keep an eagle eye on the association of terms with one another and related words in order to maintain a jump on the search engines. If SEs are not already monitoring temporal (time-based) word associations, they will be doing this very soon, and popularity (and rankings) will be directly impacted by the collage of words that are sparking interest in a topic at the given moment.
SEOmoz’s C-Index tool can be a good starting point, but additional factors are neccessary to get a true sense of word relationships. When using the tool, note that the larger the number of results returned, the less the relationship between the terms.