It is arguable that Ruby on Rails is the reason why the MVC architecture is so popular so, even if you don't use Rails, the fact that there is a release candidate of version 3.0 is important. Where Rails goes others follow.
You can try out Rails 3.0 very easily with a simple install command.
The main emphasis of the release seems to be speed improvements. As with all interpreted languages, speed is an issue and the Rails framework is quite demanding. It is claimed that speed is good across all aspects of operation apart from Active Record, and it is hoped that will be fixed before the final release.
The best way to sum these changes up is to say that if you are about to buy a Ruby book - make sure it applies to version 3.0.
The release candidate has support for the MySQL2 gem, which takes care of MySQL encoding issues on Ruby 1.9.2.It also fixes problems with autoloading and web encoding.
You can download the release candidate from http://rubyonrails.org/