I think what fascinates me most about modern computing—particularly mobile computing—is how individual people arrange specific mixtures of apps and services to suit their own lifestyle. It makes for a nearly infinite set of possibilities.
The iPhone's home screen is the new pocket-sized compact mirror. Instead of reflecting your face, it casts back the complexion of your tastes at any given point in time.
On a far more practical note, Mark Krieg is using his iPhone to survive his commute.