The logic behind the invisible layers theory though, is that if the search engines can’t read the Flash in the first place, then how can it know whether or not the text is different than the text presented to the visitor? I believe this is a very risky practice. The moral standard would be that anytime you are presenting information to a search engine that is different than the information presented to a visitor, especially if it is intended to be deceptive, it is considered cloaking; therefore, a black-hat SEO practice.
Because I truly believe after having explored the SDK option, search engines do have the capability of reading flash files, not only will invisible CSS layers duplicate your content, but also these efforts will be perceived as spam. If not right now, then it will happen soon. So my advice is to stay away from this technique, or you could likely find yourself not showing up in the search engine results pages (SERPs) at all.

Other Flash Tips:

Use accessibility features in Macromedia’s Studio MX to enhance the Flash. This will help accessibility readers purvey the content to visitors that rely upon the options in their browsers.
Make sure Flash links point to HTML pages, not other Flash pages. While the ability of a search engine to follow links in Flash is improving, it may not index content if the link just points to another Flash page.
Use static text in a Flash movie, so that these instances are not duplicated, and then are perceived as spam.
Recommend to the Flash developer that he use motion tweening as much as possible. Tweening is a term that comes from “in between”. Motion tweening is used when an animation occurs between two keyframes, and the Flash application creates all the frames in between. This also reduces the occurrence of duplicate text.
Leave the user with the ability to skip or turn off sounds and animations.
Never try to hide text within Flash.
You may have to educate your client on the downside of using a Flash site, and persuade them to use other methods of designing their website, but if you must optimize a Flash site, all hope is not lost. In fact your chances of your Flash movies being read by search engines are very good, and improving all the time. I do believe that the text in a Flash movie can indeed be read by those search engines that implement the SDK technology, whether with support from Macromedia, or with similar technology being utilized.

So can your client have their Flash site without sacrificing rank? You bet. By working with the developer, effectively communicating your strategies to both them and your client, and by understanding the technology used by the SDK, you will find that optimizing a flash site is not miraculous, but sometimes, rather worthwhile. Your client will be thrilled to know that not only can they have their cake; they can eat it too.