The CodeIgniter blog has pointed out something indicating the spread in popularity of the framework - a Japanese CodeIgniter book:

A testament to the strength and flexibility of CodeIgniter is how quickly its been adopted internationally. Today, some of the very active users at CodeIgniter Japan have released CodeIgniter Tettei Nyumon (The Definitive Guide to CodeIgniter).

You can find out more about CodeIgniter Japan from their site. You can also order yourself a copy online.