As per the announcement on the WP blog:

We would like to announce that WordPress 1.5.1.3 is now released as we continue the availablity of a highly stable and extremely popular branch based on the 1.5 Strayhorn codebase. Development has moved on to some exciting new features for the next major release, but an important security issue was brought to our attention which required an update for our users. The problem is not yet public but you should update your blog as soon as possible to 1.5.1.3. If you are unable to do upgrade in the short-term you may protect yourself by deleting the xmlrpc.php file from your WordPress directory.

I have upgraded WordPress to the 1.5.1.3 this evening.

I also noticed that my feed has not been agregating on any of the places that syndicate my blog that use Mark Pilgrim‘s Universal Feed Parser (planetN and planetSuse). I checked my feed with the Feed Validator and found some errors. So, I switched to a summary of my feed rather than feeding the entire story (at least the entire first page of the story). Now my feed validates correctly. We’ll see if the sites pick it up again.