In the 10.6 version of Address Book, there are new categories that you can choose when naming phones and emails. iPhone is an option for a phone number in the pop-up menu, and MobileMe is an option for an email.
This actually carries over to the iPhone, too. If you use the hands-free features of the new iPhone 3GS, you can now say, for instance, Call Steve Jobs iPhone. I haven't seen this documented any where.

