I am not recommending this is a tactic, I am not suggesting that at all.
This is what I did, I used one of my domains names, created a frontpage added it to google sitemaps, and referanced a CSS in another domain.
The sitemap was regulaly uploaded, and so the site was also visited by google many times a day. You can actually force a visit from google by resubmitting the sitemap.
I used IIS logs, and LAN sniffers and at no point in the first 60 days did google bots 'view' the css file.
I do think they will have to start doing this now, as this is now a major part of the look and feel / quality of the site, and it is possible to have text in the html that is not on the screen - but by my tests - more out of curisoty than anything else, they do not cross referance the two files.
Sorry about the spelling, I be on the PC all night
Graham