The NYT covers the growing number of mobile phones offering VoIP calling via WiFi:

The phones, while a potential money-saver for consumers, could cause big problems for cellphone companies. They have invested billions in their nationwide networks of cell towers, and they could find that customers are bypassing them in favor of Wi-Fi connections. The struggling Bell operating companies could also suffer if the new phones accelerate the trend toward cheap Internet-based calling, reducing the need for a standard phone line in homes with wireless networks.