What you did is proofing that google indexes hidden text. What you did not proof is how that impacts your ranking. Google will know that this text is hidden, although it is indexed and that is what is important.
I suspect there will always be a way to hide the text in a way that a search engine won't be able to detect. You can do this without applying any properties directly to the element containing text (raise other elements above it), hide it with JavaScript, or any number of such techniques.
I believe Google is moving towards graphical understanding of the text as well and it should be able to render the pages and see what is going on the page... those days will soon be over and technical excellency in SEO will talk.
What if you use custom blank fonts? Or simply detect the User Agent and serve a different CSS file?