In case it would be useful for someone else, I received the following 
answer from the development team :

--------------
Date: 18/01/2008 07:47
Expéditeur: Usaku Nakamura

Not a bug.
Date: 18/01/2008 07:47
Expéditeur: Usaku Nakamura

1.9's dl is not compatible with 1.8's.
The following code may help you:

require 'dl'
user32 = DL.dlopen('user32')
msgbox = DL::CFunc.new(user32['MessageBoxA'], DL::TYPE_LONG,
'MessageBox')
msgbox.call([0, "Hello", "Message Box",
0].pack('L!ppL!').unpack('L!*'))
---------------

Chauk-Mean.
-- 
Posted via http://www.ruby-forum.com/.