To make fring work on your iPhone you do need a jailbroken device (and no, it won't work on the iPod touch because you don't have a microphone and speaker [UPDATE 04/16/01: Unless you add a touchmod mic ...]), but this could be a pretty compelling reason for unlocking the iPhone to third-party applications, especially given that AT&T offer unlimited data on all iPhone plans.
I'm going to guess that this is the kind of thing that carriers such as AT&T aren't going to be thrilled about. In fact, I'd go as far as to say that it is exactly the reason why carriers don't like the idea of jailbroken iPhones. After all, this sort of application cuts them out of the loop with regards to monthly minutes. Fewer voice calls means fewer charges. Unlimited data and a limited number of minutes on the plan (for all plans except the $119.99 plan) means that users who jailbreak their iPhones will be able to push an awful lot of data over the network and not have to pay a cent for it. iPhone users have already shown that they have a voracious appetite for data, and a free VoIP application is only going to make things worse for the carriers.
If I were the owner of a jailbroken iPhone I'd now be on the lookout for a volley of updates from to try to make life harder of the jailbreakers. I don't expect Apple to be successful in closing the door on those wanting to set the iPhone free, but I'm sure it's possible to make the process a little trickier, and therefore put people off jailbreaking in the first place.