Simply go to Options > Domains in your WP Admin Panel and enter your domain name in the box. If you registering a new domain, they charge you 15$ per year (all inclusive), else if you already own a domain name, you can map it for 10$ per year and update your nameservers.
Any visitors to your blog’s old URL will be automatically redirected to its appropriate new URL. If you don’t like the domain name, you can always revert back to your same old subdomain. Pricing seems to be pretty reasonable to me. What’s your opinion ?
Although I don’t have a blog at WordPress.com, I really welcome this new feature and I hope WordPress.com allows bloggers to place ads on their blogs in future (Consider this as my Wishlist)
P.S Some Users have randomly been given a free domains for a year. A Handful of them rather. If you have a blog at wordpress.com check out your dashboard and if you don’t have one register yourself and you might be lucky enough to free domain..