Hopefully the same way they do now as nothing is different.
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.
The bots are definitely smarter than they were a few years ago. But, I don't think they are ready for CSS.
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. ;)