Jagex has announced that RuneScape 3 will be launched in summer 2013. Recently Jagex released a post on the website, saying that beta registration for RuneScape 3 will be available from April 3.
The beta registration closes on April 10. On April 17 Jagex plans to start beta testing their HTML 5 engine and on April 24 they will start alpha test on their new interface.
You can read more about the RS3 Beta Programme here.