Elliot Haughin has written up a post for all of those developers out there either already using CodeIgniter or wanting to use it for your application - a look at making a UTF-8 compatible site with the help of a few custom libraries and form helpers.

UTF8 allows your site to represent characters other than those in the basic english alphabet. More often than not, your CodeIgniter Application will contain methods where users can enter their name. [...] This guide assumes you are reasonably competent in installing php extensions, adding config variables to your php.ini, and using MY_ CodeIgniter overloading. If you’re not sure about any of these, please make sure you consult a professional.

You'll need to install the mbstring extension for PHP to be able to follow along with his example. He shows how to override the basic form functionality with custom functions to change the display of the form and how it handles the submitted information. He also looks at how to update the XML-RPC library that comes with the framework and the creation of a new helper to allow you to convert, check, compare and sort UTF-8 data.