The beta1 release of Omega marks the next generation in Drupal theming with responsive layouts, customizable grids all based on 960.gs standards, advanced theme caching with drastically improved performance, more solid debugging, and more features that can be possibly counted here.
The 3.x version is still highly undocumented, and the documentation at http://himer.us/omega960 still relates to the 2.x version of Omega.
The documentation there will soon be updated one page at a time. Please stay tuned for more information.
With the release of beta1, the 3.x line of Omega is now the recommended release for all new Omega based sites.