I number of people have asked me what is the difference between Version 2 and Version 3 of Google Maps API is.
Well for starters this is what Google says:
Aside from speed improvements, there has been a lot of work done to integrate mobile browsers (Phone Safari mobile and Chrome) and move to a Model View Controller (MVC) architecture. Why is this you ask?. Well, it is in recognition that the simple Desktop Browser is no longer the only option for displaying (rendering) output. The key idea is extraction of the execution components from the viewer (viewport), the controller (input management) and model (the process).
Basically this is the preparatory work for a host of new applications and services for the mobile world.
However, please note that Version 2 has not disappear. You can still use that version to test and develop on.