First Chrome and Firefox do not play nice with negative z-index.
IE picks up my negative z-index without a glitch but Firefox and Chrome ignore it completely.
I am going to stay away from negative z-index for now since it is not really supported across the board.
FYI: The page z-index default value is zero and the default z-index of any layers is 1.
Useful Websites
http://www.webdevout.net/browser-support-css
http://www.quirksmode.org/css/contents.html