In the DNS settings, www is entered as a sub domain. But what xxtoni says above is the reason why it is not a normal sub domain, it is treated as if it is present or not in the same way.
I just did a simple test on one of my older sites that has good PR and Alexa Rank and the figures for PR and Alexa rank are the same for all 4 combinations of:
But the PR for:
blog.site.com is completely different (Alexa rank the same).
So in this case blog.site.com is being treated as a different website by Google. But Alexa seems to only look at the TLD.