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I think Highland covered the basics - it will be seen as a sitewide URL change, so you need to make sure those 301s work. You could lose some small amount of link-juice, and expect rankings bounce in the short-term (although it should recover 98%). Also, it can add a lot of overhead to a site, so make sure your servers are up to it. Technically, Google says it's fine. They don't treat secure pages any differently than "regular" pages. Practically, I'm not a big fan, to be honest - it's adding a layer of complexity. Still, there are big sites that are secure and rank fine.

You can use rel-canonical on other, non-blog sites. It does help to have a one-to-one relationship of pages between the two sites. In theory, the 301-redirect is the better long-term approach, but in practice, rel-canonical seems faster and a bit more powerful in the short-term. So, David's basically saying to use rel-canonical for that initial boost and then do 301-redirects later to make it all permanent. It is a more technically complex approach.

That's a good idea and could be worth trying out. My only slight worry with this is as newer products are introduced, there is invariably a period of time when both are popular searches so I wouldn't really want to append 'blue widget 2010' content to 'blue widget 2011' during this period and possibly lose rankings. I could add 'blue widget 2008' and 'blue widget 2009' of course to the 'blue widget 2011' page, but this might look odd that 'blue widget 2010' wasn't there. I guess I was also asking if losing pages looks bad to Google - do they like to see a continually growing site?