Maile Ohye has an excellent writeup scanning various duplicate content issues caused by URL parameters on the Google Webmaster Central blog.
How can URL parameters, like session IDs or tracking IDs, cause duplicate content?
When user and/or tracking information is stored through URL parameters, duplicate content can arise because the same page is accessible through numerous URLs. It’s what Adam Lasnik referred to in “Deftly Dealing with Duplicate Content” as “store items shown (and — worse yet — linked) via multiple distinct URLs.”
Why should you care?
When search engines crawl identical content through varied URLs, there may be several negative effects:
1. Having multiple URLs can dilute link popularity. For example, in the diagram above, rather than 50 links to your intended display URL, the 50 links may be divided three ways among the three distinct URLs.
2. Search results may display user-unfriendly URLs (long URLs with tracking IDs, session IDs)
* Decreases chances of user selecting the listing
* Offsets branding efforts