Do you have a WordPress.com username? Did you register the matching blog name? If you haven’t, the blog name remains available for anyone to register.
Let’s say your username is ‘yourusername’. You have registered only the username but not a blog with the same name.
Now, if somebody registers a blog name called ‘yourusername’, the following URL belongs to that person – http://yourusername.wordpress.com/
If your ‘username’ has significance to you and you want to keep http://yourusername.wordpress.com/ as your blog address, make sure you register both username and blog name under ‘yourusername’.