It is time to update your WordPress installation! Version 2.1.1 includes about 30 bug fixes for encoding, XML-RPC, object cache, and HTML code. Download WordPress 2.1.1.
You don’t need to do full upgrade for the new versions. You can replace only changed files in the new version to your current installation. Please refer to WordPress blog for the file list.
Update: If you downloaded WordPress 2.1.1, 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. [hat tip: Evan]