On the CodeIgniter blog, there's an announcement about the release of the latest version of the framework - version 1.7.1:
CodeIgniter Version 1.7.1 has been released. This version contains many new features and enhancements, as well as nearly three dozen bug fixes. It also includes a critical security update for applications using the new Form Validation library with field arrays. For a list of all changes please see the Change Log.
If you want to just upgrade your current installation, check out their upgrade guide for the process. Want more information on the framework in general? Check out the main site, the manual, the forums or the project's wiki.