a friend of mine just got her iphone yesterday and sent me a txt message with the new phone number (she had move states etc etc).. when I went to update her contact record the number type defaulted to iphone... i had her previous number set up as a mobile. hmmmm.
Maybe Apple/ATT include something extra with the iphone to iphone texts or something? It doesn't really add any value at the moment and labeling a number as iphone or not iphone doesn't help much later as tagging a number as such for some mystery app doesn't help either, users will miss tag numbers anyway so relying on that doesnt work.