On a number of unexplained causes, launching the simplest program written in 
ruby/tk, leads to crashing ruby.

For example:
eveel{~}% cat ruby-tk-ex3.rb 
require 'tk'
root = TkRoot.new { title "Ex3" }
top = TkFrame.new(root) { relief 'raised'; border 5 }
lbl = TkLabel.new(top) do
  justify 'center'
  text    'Hello, World!'
  pack('padx'=>5, 'pady'=>5, 'side' => 'top')
end
TkButton.new(top) do
  text "Ok"
  command { exit }
  pack('side'=>'left', 'padx'=>10, 'pady'=>10)
end
TkButton.new(top) do
  text "Cancel"
  command { lbl.configure('text'=>"Goodbye, Cruel World!") }
  pack('side'=>'right', 'padx'=>10, 'pady'=>10)
end
top.pack('fill'=>'both', 'side' =>'top')
Tk.mainloop

eveel{~}% ruby ruby-tk-ex3.rb 
*** glibc detected *** ruby: free(): invalid next size (fast): 0x091f87a0 ***
======= Backtrace: =========
/lib/libc.so.6[0xb5d7e4]
{...}
b7ee5000-b7f71000 rw-p b7ee5000 00:00 0 
bf873000-bf888000 rw-p bffeb000 00:00 0          [stack]
zsh: abort      ruby ruby-tk-ex3.rb
eveel{~}% 

My system is:
    Fedora release 9 (Sulphur),
    ruby 1.8.6 (2008-06-20 patchlevel 230) [i386-linux],
    tcl 8.5.3, tk 8.5.3,
    glibc 2.8

Also, I've reported about this bug in http://rubyforge.org/tracker/?
func=detail&aid=21168&group_id=426&atid=1698