vm_opts.h contains:

  #define OPT_TAILCALL_OPTIMIZATION    0

In an interview, ko1 mentioned it should be enabled in 1.9's release,
and needed testing:

  http://blog.grayproductions.net/articles/the_ruby_vm_episode_v

  Current Ruby 1.9 (trunk) is not enabled this optimization. If you want
  to try this, please re-write that option in "vm_opts.h"
  (OPT_TAILCALL_OPTIMIZATION) and re-compile that. I think release
  version of Ruby 1.9 is enabled this optimization. I need more comments
  of it

That was a year ago, and I've not noticed much else in the way of
references to it.  Is anyone aware of it's current status?  It seems to
work, but I can't say I've tested it significantly.

-- 
Thomas 'Freaky' Hurst
    http://hur.st/