Although extremely hard to pronounce, canonicalization is a hot topic right now. Google's latest and greatest idea, canonicalization is the process of consolidating all duplicate URLs to one original canonical version. If there are a lot of URLs that lead to pretty much the same page, you're going to make the search engines work extra hard and spend a lot more time crawling all the different URLs. Often times, this means that they'll miss the important pages of your website because your crawl time is limited or too slow.
Here are some times when canonicalization is an issue: