Is VoIP through a mobile device likely to become a reality?

And in your opinion, is this likely to become a reality, or is it another "far into the future" technology?

Here is an interesting article at http://www.comsoc.org/ci1/private/2000/apr/Liao.html that explains the challenges of making this work. I would think the largest challenge would be in moving from cell to cell or tower to tower. Your IP address would have to be available on the next tower to assure you did not cause a network conflict with another phone. I believe that IPv6 will allow sufficient addresses for this to become more of a reality than the current gateway function that would be required today.