It is currently easy to hide text using CSS, everyone knows it. But do people do it?
Now, if you know Yahoo! and specifically Google, they typically will never say that they will be doing anything in the future. They typically first do and then tell, but not tell and then do.
I am honestly still confused by that statement. Well, if we block it, will it raise a red flag? If it raises a red flag, will you manually peak? Are you going to algorithmically crawl those files and look for problems if we keep them accessible to you? If we format something a certain way, but it may appear like spam, but in reality it is not, will an automated ban come on the site?
Personally, I am not worried. But these types of responses, by the search engines, can fuel a lot of questions and unnecessary worries.
As pageoneresults says in the WebmasterWorld thread:
Google has a hard enough time now dealing with html/xhtml. Parsing CSS files and determining whether something is hidden or not is not a solution. Now the bot would need to determine why that CSS exists. There are many valid uses of display:none or display:hidden.
For those who may be hiding things through CSS or negatively positioning content off screen to manipulate page content, I surely wouldn't do that with any long term projects. ;)
The penalty for getting busted using this technique I would imagine is a permanent ban. No if's, and's, or but's, you're history. You'll need a pardon from the Governor to be reconsidered for inclusion. ;)
Forum discussion at WebmasterWorld.