From the creators of i-Mode, NTT DoCoMo

The i-Mode Service Guideline
The guideline covers protocol stacks, security, user interfaces, downloading, and messaging. NTT DoCoMo also has an introduction to making i-mode content.

Mobile Media Japan, which is a great source for general news about i-Mode and basically all mobile technology in Japan.

i-Mode FAQ which is a very good i-Mode user FAQ.

Mobile Media Japan contains some useful links concerning i-Mode (the developer FAQ link seems to be broken, though).