Invisible mouse cursor with black background

[William Morris]

I am using Emacs 23.3.1 on OS-X with the background set to black.

The mouse cursor when over text is a thin I-beam, perhaps two pixels wide, 
black and white.  Needless to say it is almost invisible and in bright ambient 
light, completely invisible.  How can I change the colour and/or shape of the 
pointer (preferably both)?

I had a similar problem on the terminal and was able to replace the I-beam 
resource file with one I created myself. I'm hoping something similar is 
possible for Emacs.

Thanks
--
William Morris

GNU Emacs 23.3.1 (x86_64-apple-darwin, NS apple-appkit-1038.36) of 2011-12-13 
on bob.porkrind.org