Interesting idea, but I would be little skeptic about it:
1) If bots are able to learn how to read CAPTCHA, then why shouldn't they be able to figure out probable human visibility of a given HTML element based on given CSS? They can be taught not to fill in anything that a human being would not be able to.
2) This solution is of little elegance, to say the least. Lots of additional elements, code...
3) Wouldn't google possibly consider it "hidden content" in their spam report forms?
I like the idea that you can actually distinguish and collect/filter spam requests, though.
I've also tried Flash for CAPTCHA purposes (http://www.ranking.slupsk.pl/index.php5/rankingi/15). That requires an intelligent HTML-flash variable passing method, it has to be encrypted or organized in a particular way. Unfortunately, some browsers had problems to display ActionScript-driven dynamic SWF files, and I haven't developed another working version, yet. SWF files can also be decompiled, so it's not unbeatable, either.