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Photographers (my industry) are often setting up a blog at theirdomainblog.com

So if their name is Jane Smith, their site would be www.janesmithphotography.com and their blog would be on janesmithblog.com

What I've seen from the SEO side of this is it works well for them to have janesmithphotography as their main site and then another domain as their blog site.  However, we do find they don't easily rank for long tail keywords very quickly.  All that "domain juice" they would get from links on sites many webmasters put their link on the janesmithphotography domain and not on the blog.

So for my money (and business), I'd rather have a blog on a /blog or blog.domain.com than on its own domain.

To the posterous question - I would never SEO another company's domain.  I tell all of my blogger.com clients they need to move to their own domain or they need to hire someone who wants to do SEO for Google.  So I would HIGHLY recommend against that.

I've found that if you manually ping Google, they often update their cache at the same time.

Google doesn't have a cache for either cache: www.fuchia.co.uk. or cache: fuchia.co.uk. so I don't think it's a canonical issue.

I would suggest a few things:

1. Use PingDevice http://www.pingdevice.com/

2. Put your main domain in a Google Plus post every now and then.

3. Resubmit a sitemap.  Usually this gets you crawled fairly quickly and possibly updates your cache.

Fantastic, fantastic post Ryan!  Dealing with this for a client now - good info!