According to the Greenstone3 page, this major rewrite is "a complete redesign and reimplementation of the original Greenstone digital library software (Greenstone2). When complete, it will retain all the advantages of Greenstone2 - for example, it will be multilingual, multiplatform, and highly configurable. It incorporates all the features of the existing system, and is backwards compatible: that is, it can build and run existing collections without modification. Written in Java, it is structured as a network of independent modules that communicate using XML: thus it runs in a distributed fashion and can be spread across different servers as necessary. This modular design increases the flexibility and extensibility of Greenstone. Please note, Greenstone3 is our research version of Greenstone, and is still incomplete, and not stable. For a production digital library we recommend using Greenstone2."
Distributions of the 3.x release line are available on that page, and a source tarball of Greenstone3 is cached here also.