WordPress has recently added a feature that makes using Google webmaster tools extremely easy. I had previously posted a workaround to trick google to letting you use webmaster tools in wordpress. But the kind folks at WP have now built this into the dashboard so it can be accomplished in a few easy steps (rather than the 15 steps it used to take). I am not sure if this is the same for wordpress.org hosted blogs.
Click “Tools” on your dashboard. Toward the bottom of that screen you will see a place to paste in the meta code from google webmaster tools. Log into your google/gmail account and go to google webmaster tools. Get the meta code you need for your blog to be verified and paste it into the place in WordPress’ Tools section. Go back to Google webmaster tools and click “Verify.” It will take you right into your diagnostics. There is some very helpful information in there regarding how people get to your blog.