Buzz2Talk is a Symbian application that lets you push-to-talk and use SIP to talk over gprs. What this means is that you can use the data channel on your phone to do voice over IP instead of making voice calls. This means that if gprs becomes flat rate (IF) then you will be able to call your friends without using the carrier telephone circuits. This looks like yet another extremely useful application for mobile phones that really throws the whole billing structure of mobile phone operators through a wringer. Does voice become free? Should they charge more for different kinds of traffic on gprs? Won't people build work-arounds?
Symbian is great because it is an open platform and allows software developers to develop obvious things that carriers won't build because of their "optimized" billing structures.