The firmware 1.1.1 allows to select a region format for displaying dates, times, numbers, names of days of the week and months, phone numbers etc. This means you no longer need to manually edit UIPhoneFormats.plist. However, currently you cannot mix-and-match the various aspects of a region format. For example, if you set region to Australia, your phone numbers will be displayed in the correct Australia format, but your weeks will have to start with Sunday, and dates will have to be displayed as dd/mm/yyyy.
In order to customize a locale, you will need to edit the internal database of locale settings. They are stored in the ICU data file (/usr/share/icu/icudt34l.dat). The ICU stands for International Components for Unicode, further information can be found at http://icu-project.org/.
Some people have tried to recompile the data file, but SpringBoard seems to crash if the original icudt34l.dat is modified.