A Google Groups thread asks if there's any such penalty for using the z-index HTML attribute underneath images. Would that be deceiving the search engines if you're putting text underneath an image that is not apparent to the visitor?
Bergy from the Google Webmaster Central team says that it's not a bad practice:
So, the technique you've laid out here is neither good nor evil. ... [I]f 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.
That's interesting, and it should help other web designers who are in a similar pickle. ;)
Either way, hopefully the quality measures really do not mind.
Forum discussion continues at Google Groups.