You would just need to use the multisite functionality now in WordPress, see http://codex.wordpress.org/Create_A_Network. Setup the site to use the subdomain install. You would then setup the main blog to be your site, ignoring the posts section for that site. Then setup a new site within the Super Admin menu and set the subdomain for the new site to be blog.yoursite.com. You can then write blog posts from this new site while keeping the site navigation in the root install.
