the 10’12 key pad also has a advantage of being within the old 7-10 digit number set that is easy for us humans to remember. When you look at your 12 key pad on your keyboard you see a system that is also scaled to have large easy to use keys even with in a small foot print. I am certain that when apple did the design for ipod they said "ok whats sized for the hand, and the 10/15 key pad was laying right in front of everybody. Reinvention of the wheel can be a lot of fun for some, but in the end what has been around for a long time works pretty well or would not still be around.