Jill Whalen answers some SEO related questions in SEL Blog. My favorite is about choosing between a sub-domain or sub-directory for your company blog:

Question: Should I put my blog in a sub-directory, a subdomain or on its own domain?

Jill: It depends on: whether you want to brand the blog as part of your main website or brand it as a separate entity, and whether you want people to be able to easily remember the URL.

The quick answer: Whether it’s in a sub-directory or subdomain doesn’t make too much difference, although if it’s in a subdomain you may have more chance of it showing up in the search results at the same time that your main domain also shows up, than if it’s in a sub-directory. On the other hand, the average person doesn’t think to type in subdomains if they’re trying to go directly to your site and are more likely to remember something like yourdomain.com/blog than blog.yourdomain.com. For SEO purposes, none of these things really matter, so it’s more of a business/branding decision.

fletchgqc adds: It does have some SEO importance since a subdomain is a separate domain? If you have a lot of links coming into your blog posts, wouldn’t that give authority to your domain as a whole and therefore vouch for having your blog as a sub-folder? Alternatively if you have heaps of outgoing links from your blog and not much coming in it might be better to have it as a subdomain. That was running through my head recently but I don’t really know the answers so please feel free to enlighten me.