Robert_Charlton - 7:05 pm on Oct 31, 2011 (gmt 0)
Here's a Matt Cutts video announcing the canonical link element....
Canonical Link Element
Matt covers many issues regarding the element, frequently also called the "canonical tag". Here's a link to a transcript of the talk on Matt's blog, with clickable links to sections of the video....
Clickable transcript of my Canonical Link Element talk
Also, I suggest using site search here to find many discussions on the topic. It's clear from occasional questions on this forum that you don't want to make a mistake when using the canonical element, so do some reading and understand the issues.
Full canonicalization done with mod_rewrite is a better solution if you can use it, and will provide a sitewide fix.