The new Canonical URL tag used by Google, Yahoo and Microsoft. It helps you to setup the default URL the Search Engine should have for your page. For example the DotNetNuke Download Page is accessible through these links:

http://www.dotnetnuke.com/tabid/125/default.aspx,
http://www.dotnetnuke.com/Development/Downloads.aspx and
http://downloads.dotnetnuke.com (which 301-redirects to the first URL )

The new tag allows you now to give the search engines a hint about your favorite Url for that page.

Adding the code

to the head section of your page, search engines will recognize even the redirect(*) as default for your page. (* Cross domain redirects are currently not allowed).

The first modules for Wordpress and Drupal already exist, but what is about DotNetNuke?

It doesn't need any special module for that task.

Just open Page Settings -> Advanced Settings and place the tag inside the Page Header Tags.

:-)