Over at Google Groups, when asked whether text could be hidden under the z-layer or whether it was considered as deceiving search engines, Berghausen from the Google Webmaster Central team has an interesting comment.
"the technique you've laid out here is neither good nor evil. As Craig said, if the hidden content is a more-accessible-but-less-pretty version of the content that hides it–e.g. text behind an image containing those words–our quality measures should not mind. Of course, we suggest using the ALT and TITLE attributes of the IMG tag, which were designed for this very purpose (providing alternative text to replace images), but you are, of course, free to design your site as you see fit."
Apparently, there is more information available at:-