My daughter, among others, was over for a 4th of July cookout. After we'd had lunch and she was getting ready to go to work at her part-time job, I glanced at the iPhone and beheld the AT&T legend and TWO BARS. Quickly I dialed her mobile number, and it rang in her pocket. "Who's calling me? Oh, it's Daddy on his iPhone!"
So she was honored to be the receipient of the my first successful call from inside my house. Just as quickly, I said, "Call me back!". In a few seconds, she was greeted by the ringtone "Quack, Quack. Quack" ... I had assigned her the "Duck" ringtone ;) We hung up, chuckling.
The signal faded. She still had three bars on her Verizon phone, as did I on my Sprint Treo. So, at least once per five days, the iPhone can actually be used to make and receive phone calls.