さくです。

最小コードが作れるところまで絞れていないのですが、とりあえず報告します。
以下のように cucumber を実行すると高確率でSEGVになります。
rubyレベルのバックトレースが出るところまで到達しないこともあります。
問題なく実行できる場合もいくらかあります。
トレースが出た後、プロンプトには戻らず、Ctrl-Cでは止まらないので、外部からkill -9しています。

polyglot が require を再定義しているあたりがあやしい?

$ gem install cucumber
$ rehash
$ mkdir features # cucumberを動作させるための空ディレクトリ
$ cucumber
0 scenarios
0 steps
0m0.000s # うまくいく場合
: 何度か実行
$ cucumber
/Users/sakuro/.rvm/rubies/ruby-head/lib/ruby/1.9.1/timeout.rb:33:
[BUG] Segmentation fault
ruby 1.9.2dev (2010-03-16 trunk 26956) [x86_64-darwin10.2.0]

-- control frame ----------
c:0022 p:---- s:0069 b:0069 l:000068 d:000068 CFUNC  :caller
c:0021 p:0083 s:0066 b:0066 l:000065 d:000065 CLASS
/Users/sakuro/.rvm/rubies/ruby-head/lib/ruby/1.9.1/timeout.rb:33
c:0020 p:0009 s:0063 b:0063 l:000062 d:000062 TOP
/Users/sakuro/.rvm/rubies/ruby-head/lib/ruby/1.9.1/timeout.rb:25
c:0019 p:---- s:0061 b:0061 l:000060 d:000060 FINISH
c:0018 p:---- s:0059 b:0059 l:000058 d:000058 CFUNC  :require
c:0017 p:0015 s:0055 b:0055 l:000054 d:000054 METHOD
/Users/sakuro/.rvm/gems/ruby-head%polyglot/gems/polyglot-0.3.0/lib/polyglot.rb:65
c:0016 p:0071 s:0048 b:0048 l:000047 d:000047 TOP
/Users/sakuro/.rvm/gems/ruby-head%polyglot/gems/cucumber-0.6.3/lib/cucumber/step_mother.rb:6
c:0015 p:---- s:0046 b:0046 l:000045 d:000045 FINISH
c:0014 p:---- s:0044 b:0044 l:000043 d:000043 CFUNC  :require
c:0013 p:0015 s:0040 b:0040 l:000039 d:000039 METHOD
/Users/sakuro/.rvm/gems/ruby-head%polyglot/gems/polyglot-0.3.0/lib/polyglot.rb:65
c:0012 p:0136 s:0033 b:0033 l:000032 d:000032 TOP
/Users/sakuro/.rvm/gems/ruby-head%polyglot/gems/cucumber-0.6.3/lib/cucumber.rb:7
c:0011 p:---- s:0031 b:0031 l:000030 d:000030 FINISH
zsh: killed     cucumber

$ cucumber
cucumber
/Users/sakuro/.rvm/rubies/ruby-head/lib/ruby/1.9.1/timeout.rb:33:
[BUG] Segmentation fault
ruby 1.9.2dev (2010-03-16 trunk 26956) [x86_64-darwin10.2.0]

-- control frame ----------
c:0022 p:---- s:0069 b:0069 l:000068 d:000068 CFUNC  :caller
c:0021 p:0083 s:0066 b:0066 l:000065 d:000065 CLASS
/Users/sakuro/.rvm/rubies/ruby-head/lib/ruby/1.9.1/timeout.rb:33
c:0020 p:0009 s:0063 b:0063 l:000062 d:000062 TOP
/Users/sakuro/.rvm/rubies/ruby-head/lib/ruby/1.9.1/timeout.rb:25
c:0019 p:---- s:0061 b:0061 l:000060 d:000060 FINISH
c:0018 p:---- s:0059 b:0059 l:000058 d:000058 CFUNC  :require
c:0017 p:0015 s:0055 b:0055 l:000054 d:000054 METHOD
/Users/sakuro/.rvm/gems/ruby-head%polyglot/gems/polyglot-0.3.0/lib/polyglot.rb:65
c:0016 p:0071 s:0048 b:0048 l:000047 d:000047 TOP
/Users/sakuro/.rvm/gems/ruby-head%polyglot/gems/cucumber-0.6.3/lib/cucumber/step_mother.rb:6
c:0015 p:---- s:0046 b:0046 l:000045 d:000045 FINISH
c:0014 p:---- s:0044 b:0044 l:000043 d:000043 CFUNC  :require
c:0013 p:0015 s:0040 b:0040 l:000039 d:000039 METHOD
/Users/sakuro/.rvm/gems/ruby-head%polyglot/gems/polyglot-0.3.0/lib/polyglot.rb:65
c:0012 p:0136 s:0033 b:0033 l:000032 d:000032 TOP
/Users/sakuro/.rvm/gems/ruby-head%polyglot/gems/cucumber-0.6.3/lib/cucumber.rb:7
c:0011 p:---- s:0031 b:0031 l:000030 d:000030 FINISH
c:0010 p:---- s:0029 b:0029 l:000028 d:000028 CFUNC  :(null)
c:0009 p:0023 s:0025 b:0025 l:000024 d:000024 TOP
/Users/sakuro/.rvm/gems/ruby-head%polyglot/gems/cucumber-0.6.3/lib/cucumber/cli/main.rb:2
c:0008 p:---- s:0023 b:0023 l:000022 d:000022 FINISH
c:0007 p:---- s:0021 b:0021 l:000020 d:000020 CFUNC  :(null)
c:0006 p:0082 s:0017 b:0017 l:000016 d:000016 TOP
/Users/sakuro/.rvm/gems/ruby-head%polyglot/gems/cucumber-0.6.3/bin/cucumber:5
c:0005 p:---- s:0013 b:0013 l:000012 d:000012 FINISH
c:0004 p:---- s:0011 b:0011 l:000010 d:000010 CFUNC  :load
c:0003 p:0127 s:0007 b:0007 l:000e18 d:0015e0 EVAL
/Users/sakuro/.rvm/gems/ruby-head%polyglot/bin/cucumber:19
c:0002 p:---- s:0004 b:0004 l:000003 d:000003 FINISH
c:0001 p:0000 s:0002 b:0002 l:000e18 d:000e18 TOP
---------------------------
-- Ruby level backtrace information ----------------------------------------
/Users/sakuro/.rvm/gems/ruby-head%polyglot/bin/cucumber:19:in `<main>'
/Users/sakuro/.rvm/gems/ruby-head%polyglot/bin/cucumber:19:in `load'
/Users/sakuro/.rvm/gems/ruby-head%polyglot/gems/cucumber-0.6.3/bin/cucumber:5:in
`<top (required)>'
zsh: killed     cucumber

-- 
OZAWA Sakuro

"The future will be better tomorrow." --Dan Quayle