No SEO hit with the upgrade - it costs $12.00 per year to take up the option.
Hello, Goodbye: Offsite Redirect Upgrade
Needless to say, we hope you never want to leave WordPress.com. But, should the day arrive that you decide to strike out on your own and set up a self-hosted WordPress blog with the free, open source WordPress software available at WordPress.org, we’ve got you covered with our new Offsite Redirect upgrade.
Imagine you’ve decided to make the move, have exported your content from your wordpress.com blog and imported it into your new self-hosted WordPress site. Purchasing the Offsite Redirect upgrade will take the traffic coming to your old wordpress.com blog and forward it to your new domain so that links to your old content won’t break and you won’t take an SEO hit as a result of your move. Oh, and unlike the old domain mapping/DNS workaround, the signup process is easy as pie. Mmm, pie.
Read more at en.blog.wordpress.com