There is no Google Talk application for the iPhone, and if there is one, I seriously doubt it has any video calling feature. For cross-platform, you can try Tango or Skype (not sure if either support ICS at this time - but you can check if you want).
I use Google Talk from Galaxy Nexus to Galaxy Nexus or Xoom/Tab 10.1, and it works great between all 4 of my devices. A bit choppy on 3G, but when both are on Wi-Fi (different Wi-Fi connections, that is) it's pretty smooth and has a lot less voice/video artifacts. Try it!