Back in 2007 there was a lot of talk about VoIP company Jajah and their attempts to bring a client to the iPhone; in the end, AT&T's definition of "conference call services" scuppered those plans and Jajah instead launched an iPhone-formated web interface for their ring-back service.  With the arrival of the official Apple SDK, however, Jajah are now apparently working on a native VoIP client for the iPhone, thanks to Cupertino's blessing that software which routes voice calls over the internet will be permitted on the handset as long as it uses WiFi, not EDGE, for their connection. 

Rather cleverly, the Jajah client will use both VoIP and the existing call-back system to get around that Apple WiFi restriction.  Calls placed when in connected via WiFi (such as in the home, office, or when using a hotspot somewhere) will be routed over the internet; those initiated when in a low-bandwidth situation (e.g. when only EDGE is available, or when the WiFi connection is too slow for satisfactory VoIP use) will default to the Jajah call-back system.

The app is expected sometime during the Summer, around the release of iPhone Firmware 2.0.

[via SlashPhone]