The VoIP app for the iPod Touch has now been released. You can view a tutorial on how you can add this App to your iPod Touch by following the below link:
http://addvoipipodtouch.notlong.com
Unfortunately you can only preorder the microphone for the iPod Touch, so right now you can only dial, not talk!
Unless you can make your own microphone for the iPod Touch and find one. TouchMods.net is the only place I found that has one, but it's not out yet.