mri-2.1.4

2.1.4 :003 > require 'yaml'
 => true
2.1.4 :004 > Psych.dump({fu: 'bar'})
 => "---\n:fu: bar\n"

rbx-2.2.7

rbx-2.2.7 :003 > Syck.dump({fu: 'bar'})
 => "--- \n:fu: bar\n"
rbx-2.2.7 :004 > YAML.dump({fu: 'bar'})
 => "--- \n:fu: bar\n"

rbx-2.4.1-with-syck

require 'sycrbx-2.4.1 :001 > require 'syck'
 => true
rbx-2.4.1 :002 > require 'yaml'
 => true
rbx-2.4.1 :003 > Syck.dump({fu: 'bar'})
 => "--- !ruby/hash-with-ivars\nelements:\n  :fu: bar\nivars:\n  :@entries: !ruby/object:Rubinius::Tuple {}\n  :@capacity: 16\n  :@size: 1\n  :@max_entries: 12\n  :@state: &1 !ruby/object:Hash::State\n    compare_by_identity: false\n    head: &2 !ruby/object:Hash::Bucket\n      key_hash: 2799345636274088058\n      value: bar\n      link: \n      state: *1\n      key: :fu\n    tail: *2\n  :@mask: 15\n"

rbx-2.4.1-with_psych

rbx-2.4.1 :004 >   require 'yaml'
 => true
rbx-2.4.1 :005 > Psych.dump({fu: 'bar'})
 => "--- !ruby/hash-with-ivars\nelements:\n  :fu: bar\nivars:\n  :@entries: !ruby/object:Rubinius::Tuple {}\n  :@capacity: 16\n  :@size: 1\n  :@max_entries: 12\n  :@state: &1 !ruby/object:Hash::State\n    compare_by_identity: false\n    head: &2 !ruby/object:Hash::Bucket\n      key_hash: 1350165056510179320\n      value: bar\n      link: \n      state: *1\n      key: :fu\n    tail: *2\n  :@mask: 15\n"