Enabling more international keyboards than the ones supported by default in 1.1.2 on a US iPhone turns out to be quite simple. All it takes is editing the file /var/root/Library/Preferences/.GlobalPreferences.plist with your favorite plist editor. By default, this file contains the following.
In order to enable another keyboard just add the relevant entry below the en_US entry. For example, to enable the Japanese keyboard, modify the above section to look as follows.
Voila! You'll now be able to enter Japanese in all applications just like our iPod touch brethren.
NOTE: Edits made to this section of the .GlobalPreferences.plist are reset if you go into either Settings -> General -> International or Settings -> General -> Keyboard. Therefore, make all necessary International and Keyboard settings before editing the file.