WordPress released a new version 2.1.1 which is mostly bugfixes and security fixes. Version 2.1.1 includes about 30 bug fixes, mostly minor things around encoding, XML-RPC, the object cache, and HTML code. Here is the file diffs between v2.1 and v2.1.1. (No guarantee comes with this). Download WordPress 2.1.1 upgrade diff. Just overwrite the existing files, upload them, then run the upgrade program.
Read about the upgrade in depth on the WordPress blog.