source : http://seorank.byeasynet.com/gerer-les-depassements-de-texte-dans-une-div-avec-une-scrollbar/

All these statements suggest that Google does try to detect intent, and is not going to ban a site solely because of someone using hidden text in a way that appears to be legitimate. This does open the door to those who want to abuse this. If someone stuffs a few words in a bit lf legitimate looking text here or there, it's hard to detect algorithmically. However, this is a trap door and an accident waiting to happen. Many webmasters who choose to walk the line on this technique may well be walking the line on other techniques. Google, and the other search engines, relay on this to out real abusers. Also, competitors are anxious to expose those sites that are over the line.

Witness the commentary in my recent interview with Matt Cutts. We talked about a blog post that a relatively little known blog about a competitor ranking for the term access panel using hidden text. Matt Cutts had picked up on this quite quickly, and Google was prepared to take action on it. However, it turns out that the site that was "outed," responded and removed the hidden text, so as Matt indicated in our interview, he removed the offending text. The point is that your competitor wants to report you for doing bad things. That motivation should be a strong deterrent to abusing these techniques.

Ultimately, intent is one of the most important factors. Don't use these techniques to abuse the system. Too much of a good thing turns into a very bad thing. Also, use them in commonly used ways. This is no time to invent some novel new way to apply hidden text to making your site design snazzy or better. For better or worse, doing something unusual, even if your intent is pure, is just asking for trouble. While the search engines want to treat your site appropriately, you make it harder for them by inventing new and unusual coding techniques. Stick to the methods that are commonly in use by others, and you will be better off. In addition, even if your use is completely legitimate, you still need to use any hidden text techniques in moderation. Extensive use of any technique, even in perfectly legitimate ways exposes you to risk. This may by wrong or unfair in some ways, but it's the world we live in. Being morally right, but banned, does not help anyone at the end of the day.