REE is based on MRI Ruby 1.8.7, which is also being phased out (1.8.7 will receive regular maintenance until June 2012, and security fixes until June 2013). REE contains patches that address the following shortcomings of Ruby 1.8.7 to make it perform better under typical server-oriented usage scenarios. A copy-on-write friendly garbage collector to reduce memory usage in Rails applications. So even though REE is being phased out, Phusion will continue releasing new versions. The latest release, 1.8.7-2012.02, upgrades the underlying Ruby to 1.8.7-p358, RubyGems to 1.8.15, contains the latest MBARI patches, and is now compatible with Xcode >= 4 and OS X Lion. Read more on this topic here.