WordPress.com (the blog hosting platform) was compromised by hackers using an undisclosed vulnerability. My guess is the attackers found an unpatched server somewhere, and used that to get into the environment. Information from Automattic is limited, but they’re assuming that source code and other information was probably stolen. Nobody has come forth to claim the hack, or post WordPress’ source code and account information online, Gawker-style.
If you have a blog on WordPress.com, I recommend changing your password there (and on any other site where you may have used the same password). If you host your own WordPress blog, there isn’t cause for concern just yet as there are many ways that the hackers could have gotten root access, so the vulnerability used may not be within the WordPress software itself.
I’ll update this post should any more information come to light.