I rarely use my Cell phone for anything other than to call someone. Its turned on just before I make the call, and then immediately turned off after I've finished. But using it for this purpose makes total sense..
I was at Wal-Mart last night.
Cashier: "That'll be $19.45."
Me: pulls out a $50 bill,
Cashier: "Sorry, we've been having a problem with counterfeiting... have anything smaller?"
Me: pulls out a $25 bill
Cashier: "Thanks, I'll get your change."