Honest webmasters should not loose sleep thinking that Google might penalize their websites one fine morning without giving any reason. Register your website with Google Sitemaps and Google may send you an email if you site is removed from Google index but could be legitimate. You can submit a site reinclusion request within the Sitemaps Control panel.

However, there is equally bad news for spammers. Matt Cutts has clearly said that Google will penalize and throw websites out of their index without any prior warning if the webmasters don't confirm to the Google Webmaster Guidelines.

Keyword Stuffing, deliberately misspelling words or hiding text from users which is otherwise visible to search engines - Any such tricks to game Google could lead to an immediate ban with no warning. And Google does have lot of resources now as Matts' said:

In the early days when Google had 200-300 people there was no way we could do everything we wanted to do. But as Google grows, we get more of a chance to “go back and fix things,” to build the ideal search engine.

Regarding CSS display: none property, Matts writes that if display:none is used to hide text, that can cause issues.

While reading Matt's entry, I did discover one website which could immediately invite a Google ban. Look at these screenshots.

This is how a user will see the website. Notice the empty black area at the bottom.



And this is how the Google spider would see the website - the color of stuffed keyword text is same as that of the page background and hence that text is invisible to the user but Google spider would read it.



The hidden text becomes visible when you try to select that area with a mouse.