While my brother is playing around with GTalk video/audio calls I came to a point, where I believe I need to buy an adroid-powered phone :)
Not sure if the functionality I want is already there, but I believe it is on a way. I am talking about transparent selection of transportation media for a phone call to a friend. Imagine an android phone with WiFi and GTalk integrated (not even talking about GMail and Contants stuff). So with such thing when you want to make a call to your friend, the phone will check if you are on Internet and your friend is signed in to GTalk and try calling for free through this channel, otherwise just make a normal dial through GSM. Manual control should also be present for sure (so a user can decide himself how to call).
On the other hand, if your friend has a similar mobile, he can receive GTalk voice/video calls directly on it as well. So phone-to-phone call is free through Internat exactly as it would be through GSM.
Ofcourse there are plenty of things to consider before the idea could become completely usable, but as an idea, I think it is really cool.