Once, it was a given, that it did not matter where your blog pots were, it was your choice... folder or subdomain.
There are a couple of decisions to be made. If you used a subdomain, then you have always had the potential of ranking twice for some keywords. This can be advantageous when you might want to rank twice for your brand, or a particularly appropriate keyword.
A key disadvantage is that wordpress, or as my developer puts it "wordmess" is safer on a subdomain. As the most popular CMS in the world, it is a target for hackers, and even with a plugin like wordfence, it is still better put on its own subdomain.
Now that Google have changed the way they view sub domains, it is a more difficult decision.
Simply, they view a subdomain as a completely separate domain in its own right. This is because there are a number of people that resell a part of their domain. wordpress is one. So while it is the right think to do, it makes it difficult if you chose to put your blog, your press section, or your regions in subdomains instead of in folders.
It might be of interest to you that in a recent conversation with Eric Ward, he agreed that he could only think of people having trouble moving from folders to subdomains in the last 12 months, but no one having ranking problems going the other way.
A great thread on this is Good Ol' Moz.
