WordPress.com has gown down apparently taking down millions of blogs with it. We were one of them too, however, we are not hosted on WordPress servers, but were using a plugin called WordPress Stats for analytics purpose.
However, our site was down and out for almost half an hour, the reason apparently because we used the WP Stats plugin.
So if your site is down and you have no clue why, just login to FTP and delete the WP Stats plugin and everything should work fine again. Let me know if your site has gone down after WordPress.com went down.
Update: You might see downtime on your site if some of your plugins try to access WordPress.com, this might not just be for the WordPress Stats plugin.
Update: WordPress.com looks to be back up again.
Update: Source was The Next Web