FINAL UPDATE: this turned out to be a false alarm! do not panic!! self-referential canonical link is fine, and doesn’t cause problems. You can still read the post and comments below to see how I got confused.
After solving a problem for a client, I looked into newest release of the WordPress plugin All-in-One-SEO-Pack, and have found a bug in it which caused the same problem that client had on a plain HTML site. It implements incorrectly recently invented Google Canonical link rel attribute, which results in pages being excluded from the index.
It should not be implemented on the valid pages that you want to have indexed, but only on redirects…
My prediction is that people’s websites will start disappearing from Google index due to incorrect use of this attribute.
Although Google claims that ‘Our algorithm is lenient: We can follow canonical chains…‘, (http://googlewebmastercentral.blogspot.com/2009/02/specify-your-canonical.html) which may be true for proper implementation, I think they should have foreseen this common problem.
Anyhow, I bet if you use this tag improperly, you won’t be able to find your pages indexed in Google — pages which have something like this in the HEAD section:
<link rel="canonical" href="http://www.example.com/my-current-page-being-indexed/" />
or something like this in case its an index.php/html/htm… type of page:
<link rel="canonical" href="http://www.example.com/" />
If you do find them, look at cached version, and you will see its an older version of the page you had before you implemented this attribute.
Update: even this page didn't get indexed in Google as All-In-One-SEO-Pack is active on this blog. All you can find in Google is the url with anchor text as title, and description surrounding the link on the main blog page.
Update2: following up on the first comment below from, here is a screenshot:
UPDATE 3: read the comments below to see what happened when canonical was removed and reintroduced to this blog.