CainIV - 5:40 am on Apr 15, 2010 (gmt 0)
Whenever you have a product type or category type that offers any kind of variations, and those variations are expressed as new pages, the canonical tag should be used.
Additionally, if multiple pages are generated to show products in one category or one explicit type, and the result is a paginated list, the pages should be tagged.
I always use the original page as the page to reference for the canonical link tag. For example, if I have the following
Then I would setup the link canonical tag to point to the full url + /my-product/ (the other pages are only color variations)
Hope this helps