"(...) The specific methodology for this tool would be to first take the average volume of searches related to a brand or topic within a period of time, potentially a rolling two week period. Then compare this number to the total volume of searches applied to a specific category, such as soft drinks or movies, and compare this to the increase/decrease in total average searches for that period of time. (...)"Sites like Google's Zeitgeist or Yahoo's Buzz-Index are already doing something like this, but on a very limited basis: only for the top search topics.
Cory is absolutely right: being able to track the search activity for a certain keyword over a period of time would definitely be extremely helpful, for example to continuously measure the impact of an ad or PR campaign. One wonders why Google or Yahoo haven't come up with such an idea years ago.
But on the other hand, there are already quite a few interesting tools available, which can perform similar tasks - through the power of weblogs !
If something becomes a hot topic on the Internet today, weblogs are very likely to pick it up. So instead of asking what people are searching for, just keep track of what Bloggers are writing about.
Luckily this is an easy thing to do: Blog-Indexes like Blogdex or Popdex are constantly showing what weblogs are buzzing about. And specialized search engines like Technorati or Feedster allow searches for postings containing certain keywords.
Due to the nature of weblogs and their authors, this will probably not work with every product or target group. But especially for viral marketing campaigns, which try to use the Internet as a way to distribute the viral agent, using the weblog infrastructure should be a no-brainer.
By the way: This tactic is also a good argument for setting up microsites with nice looking URLs to support marketing campaigns, because they are much easier to link to and to keep track of. Framed webpages, buried deep within CMS-driven corporate websites won't do the job !!