A handful of geeks have managed to make what appears to be the first iTouch call. They relied on a SIP (Session Initiation Protocol) application developed for the Nintendo DS gaming console to conduct the hack.
The user interface is very rudimentary and exposes the raw innards of the iTouch operating system. The software used SvSIP, an application developed by a French developer, Samuel Vinson, which relies on SIP.
SIP is an open standard for creating, modifying, and terminating sessions with one or more participants and It is widely used as a signalling protocol for Voice over IP.
Right now, the application running on the iTouch does not make use of any Graphical User Interface, but this seems to be the next logical step, especially as there are already at least a dozen existing numeric keypads for the Nintendo DS SvSIP.
As yet, there are no plans to turn this home-brewed application, but the iTouch could suddenly turn into a rival for the iPhone, something that Apple surely hasn't foreseen.