gauss:hydra brian$ rvm install --branch hydra rbx-nhydra
rbx-head-nhydra installing #dependencies
Cloning git://github.com/evanphx/rubinius.git
Branch hydra set up to track remote branch hydra from origin.
Copying from repo to source...
rbx-head-nhydra - #configuring
rbx-head-nhydra - #compiling
rbx-head-nhydra - adjusting #shebangs for (erb ri rdoc).
rbx-head-nhydra - #importing default gemsets (/Users/brian/.rvm/gemsets/)
gauss:hydra brian$ rvm list

rvm rubies

   rbx-head [ i386 ]
   rbx-head-nhydra [ i386 ]
   ruby-1.8.7-p334 [ i386 ]
=> ruby-1.9.2-p180 [ i386 ]

gauss:hydra brian$ rvm use rbx-head-nhydra
Using /Users/brian/.rvm/gems/rbx-head-nhydra
gauss:hydra brian$ which ruby
/Users/brian/.rvm/rubies/rbx-head-nhydra/bin/ruby
gauss:hydra brian$ ruby -v
rubinius 1.3.0dev (1.8.7 33c8117b xxxx-xx-xx JI) [i686-apple-darwin9.8.0]
gauss:hydra brian$