If you haven't seen it yet, here it is. It's been known for about a week now.
From the article:
The exploit relies on a bug in the implementation of OpenSSL’s “heartbeat” feature, hence the “Heartbleed” name. Security firm Codenomicon has written an in-depth breakdown of the Heartbleed bug here.
To quote their findings:
We have tested some of our own services from attacker’s perspective. We attacked ourselves from outside, without leaving a trace. Without using any privileged information or credentials we were able steal from ourselves the secret keys used for our X.509 certificates, user names and passwords, instant messages, emails and business critical documents and communication.