This might be worth looking into when it hits.

An open-source effort via TouchMods will bring a VoIP/SIP program to the iPod touch come New Year's Day. Right now, the program looks to be functional but still in a relatively early stage as the people involved are essentially rigging up their own microphone appendage and letting the software handle the task.

The video isn't the most exciting thing in the world, but they've got something that works and happily demo it:

Now if they can make it work under the iPhone firmware 1.1.3 when it's released...

As always, your two cents are appreciated over in the comments and forums.