Home of the Soul Cookie

If you downloaded and installed the recent 2.1.1 version of WP, it is critical that you upgrade to 2.1.2 immediately.

Long story short: If you downloaded WordPress 2.1.1 within the past 3-4 days, your files may include a security exploit that was added by a cracker, and you should upgrade all of your files to 2.1.2 immediately.

Full details over at the WP DevBlog.