Even if they can't read it, I would guess that the fact that it's hidden from them would tell them there's a reason you don't want them seeing it.
If I were a programmer at G, I'd be pretty suspicious of being locked out of a css file. I'm not sure how I'd handle it, though. If you punish a site for it, and it turns out to be an accident, then it would be unfair. Also, some sites have multiple css files and may want to restrict access in order not to confuse anyone.
maybe what I would do is load and index the site as if there was no CSS file. This would remove the ability to stuff it full of keywords (well, you still could, but it wouldn't matter).
The negative side to that would be that then you may find the css being used for hidden text.
Tough call. Maybe I'd load it and render the page, but not index or cache the CSS itself. After all, compliance with the norobots is voluntary, so it's up to the SE to either ignore it, ignore the css, or ignore your site...
Or maybe pretend that no one would ever misuse css to trick a SE. Not that I'd expect that option...