Ruby 2.0 is a not a "start from scratch" project. It's an evolution of the language (which will be done mostly in the experimental 1.9 releases) with a new interpreter. Once 1.9 is stable it will turn into the 2.0 release. The actual effect on Ruby users will be noticeable, but minimal.
with one clarification... (last I checked) 1.9.1 will be the stable, it won't be incremented to 2.0. Matz decided against the even/odd = stable/dev versioning scheme.
See http://eigenclass.org/hiki/Changes+in+Ruby+1.9 for some of the changes that are coming up.