The idea is that developers can build mini-applications that can allow real-time user interaction through the instant messaging foundation of Google Talk IM. Lettvin showed off an example of a chess game that was not only discussed over IM but played through an interface with IM-like communication infrastructure running underneath it. That's pretty hot.
University of Amsterdam student, Sander Dijkhuis, already built a collaborative text editing gadget using one of the APIs and we expect to see more interesting examples emerge shortly. Just picture ease of development plus real-time communication across different installs of little applications. There are lots of things that APIs like that could make possible.