here is some links that will help you out http://www.mattcutts.com/blog/seo-advice-url-canonicalization/ http://www.thesearchguru.com/blog/what-is-a-canonical-issue-and-how-to-fix-a-canonical-issue/ http://www.seomoz.org/article/bg4
I have also this question Can anyone explain me what is the canonical issue in a Website ? what difference between this URL http://www.mobilerainbow.co.uk/ and this http://mobilerainbow.co.uk/ ?
canonical issue is not a big issue. But if you want your site to be a perfect site as per as the search engines standards you have to follow all these small issues too. Actually google will treat the http://site.com and http://www.site.com as two different issues and this would create problems at the time of your promotion. If you are promoting the site in both the ways then you are promoting two different url. So its better to use 301 re-direct to point your site to http://www.site.com This is the standard of the search engines to be followed by the sites.
i have no any idea about Canonical Issue but today i learn something new about SEO and about issue thanks all DP members