It was just announced on the WordPress development blog that 2.6.2 has been released. If you allow user registration, it’s highly recommended you upgrade immediately.
There’s a new version of WordPress out now, 2.6.2, which addresses a database issue as well as the weakness of mt_rand(). This is especially important if you accept registrations to your blog.
Anyway, go ahead and upgrade WordPress ASAP, especially if you have open registration enabled.