I attended David Geary’s talk on Killer Web Apps with JSF 2 Ajax at JSF Summit where he demonstrated Ajax features in JSF 2. As you start with JSF 2, you will notice that many Ajax features in JSF 2 are very similar to what RichFaces has been providing for a number of years now.
RichFaces 4 will be based on JSF 2 and will provide some interesting advanced features on top of standard JSF 2 Ajax. Some features are advanced queues, advanced rendering features and tags such as a4j:poll, a4j:status and more. Here is a blog entry that demonstrates Ajax features in RichFaces 3.3.x, JSF 2 and RichFaces 4.