Did you realize that search engines have gone full circle on URLs in variables? It used to be considered something to avoid, now search engines are saying variables in URLs are good, as long as you use the canonical meta tag. Google is pushing them with FeedBurner and if webmasters aren’t careful, they could fall victim to a new onslaught of duplicate content issues.
One of the biggest issues with SEO is duplicate content. If search engines can’t tell which version of a document is the original or canonical version, then there can be consequences involving less than ideal search visibility. For example, the following URLs might all point to the same web page, creating the illusion that they are copies of the same thing. But in reality, it’s just one web page.