tedster - 6:19 pm on Dec 14, 2011 (gmt 0)
I never understood the relatively common doubt about this. Maybe because people are thinking that the canonical link "is" a 301 redirect - it's not, but Google often treats it that way when a non-canonical URL is requested.
If someone reaches a URL that is also considered the canonical address for that bit of content, then presence of a canonical link element is a simple confirmation. If the URL was varied in some way (and on most servers there are many ways to vary a URL and still get the same content) then the canonical link is doing some heavier lifting.
I would say it's valuable to stress test your canonical link script before going live - just in case the configuration is doing something funny. Incorrect canonical link tags can generate extended problems.
That said, I also realize that Bing's webmaster communicator Duane Forrester said that Bing DOES consider it an error for a URL to contain a self-referring canonical link. However, Microsoft has been trying to rewrite standard web technology for many years, introducing their own vocabulary terms, teaching soft-404s in their manuals, etc. etc. In earlier versions of the IIS server it was very hard to figure out 301 versus 302 redirects - because the GUI didn't explain it.