This is a little change in the iPhone 3.0 release that I hadn't seen anyone mention. When you change the type of a phone number in Address Book on the iPhone you've always been able to set custom labels in addition to the built-in ones. However, there was a new one added in 3.0 for "iPhone" (see 2nd choice).
Kind of odd. Not real sure what you would use this for other than vanity. I guess you could sometime specify, particularly with the coming use of MMS, that I only want to send this to iPhones?