WordPress 2.5.1

WordPress 2.5.1 Security Release

WordPress 2.5.1 has just been released. According to the development blog:

It includes a number of bug fixes, performance enhancements, and one very important security fix. We recommend everyone update immediately, particularly if your blog has open registration. The vulnerability is not public but it will be shortly.

Download it now

The question I have is: does this mean 2.3.3 is no longer safe?