Down Under, we have 10 digit phone numbers, for both Mobile (Cell) phones and Land Lines, However it's more common to see mobile numbers formatted 4 digits - 3 digits - 4 digits and landlines 2-4-4 . The "Auto Format Phone Numbers" option takes care of one of these ( by adding ## #### #### as a format option), but until now, I thought the numbers had to be of different lengths, forcing mobile numbers to be formatted like land lines. Until ...
All mobile numbers in Australia start with the prefix 04, if you add a new format into Address Book, say 04### ### ###, it will format numbers starting with 04 that way, and any other 10 number will be formatted in the other way.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040721212522850