We run a large news site and a number of much smaller news sites. The smaller sites will frequently carry content that comes from the large site on their own pages. For example
Big News Site http://www.bigsite.com/news/2011/07/27/rin_tin_tin_saves_boy/
and on our smaller site http://www.middleofnowhere.com/pets/2011/07/27/dog_saves_local_boy
The actual stories are the same, but the rest of the pages are different, including header, widgets, layout, etc.
Our boss is really into SEO and heard about canonical tags. So he wants us to make this shared content point to the main site in order to bump its ranking.
<link rel="canonical" href="http://www.bigsite.com/news/2011/07/27/rin_tin_tin_saves_boy/" />
Is this a good idea? Why or why not? Google says that canonical pages "should be similar", but not necessarily the same. Is this example
1) A canonical page
2) Not
3) Subject to debate?
When you host the same content on two sites it is correct to use canonical between the two to show which has the original article. It is the text and content of the page that need to be the same. In this case, the article. The site theme around it is irrelevant.
Google can tell when content is duplicated on the web. Even when you don't say where the content comes from (by say using the canonical tag) they figure it out by implementing a shingling algorithm They do penalize sites that do not write original content of their own, but rather take all their content from other places. However, Google has said that they allow some amount of content syndication, especially when a website acknowledges its sources. Using rel=canonical should help you avoid penalties for "scraping" content by showing Google that you are using syndicated content in a legitimate way.