tedster - 12:16 am on Nov 23, 2010 (gmt 0)
You can resolve the problem easily with a 301 redirect to the URL without the UTM query string - and/or canonical link tag in your pages' <head> section - and/or a robots.txt Disallow rule for those query strings.
However, I don't think the situation is going to cause you any ranking or traffic problems, even as it is. The information in Webmaster Tools is there "for your information" and not as a notice that you have some dire problem - notice that it's listed under HTML "Suggestions". In this case, thousands, maybe millions of websites have the same situation.