Google seems to allow hidden content in some cases

"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."