If you prefer to not use the gem but to get the 'latest svn HEAD version'...

 svn co http://rubyeventmachine.com/svn/
 cd svn/trunk/ext
 ruby extconf.rb
 make # chugs, spits out warnings
 cd ../lib
 irb
 >> require '../ext/rubyeventmachine'
 => true
 >> require 'eventmachine'
 /opt/local/lib/ruby/gems/1.8/gems/eventmachine-0.10.0/lib/rubyeventmachine.bundle: warning: already initialized constant LoopbreakSignalled
 => true
 >> EM::VERSION
 => "0.10.0"

Viola. latest and greatest.