Two developers named Arlo and Andy have developed iCall, a VOIP for the iPhone that allows for seamless swtiching to WiFi networks where available.

iCall Features

iCall is currently in beta, but offers the following features:

• Make and receive calls over WiFi
• Transfer inbound calls from your regular cell minutes to WiFi instantly
• Access your same address book
• Customize your voicemail options

More information on iCall can be found at the developer's site.

Source: EngadgetMobile.