dusky - 6:40 pm on May 13, 2010 (gmt 0)
I have a site which has the same problem, what I think it is besides blocking CSS on your robots.txt file, it may be that you have your style sheets file in a folder, example: themes/mytheme/style.cess this is fine when you have a reference to it from your pages as <link rel="stylesheet" type="text/css" href="themes/mytheme/style.cess" /> (relative path) for users or bots who are on site, but if fetching it remotely, the relative path does not have a domain name and it can't be fetched, so for previews I think, you have to write it like this: <link rel="stylesheet" type="text/css" href="http://mysite.com./themes/mytheme/style.cess" /> with full URL just like if distributing images on other sites.
The same problem may also be for images. Remember also if you fix it as above, you need to wait for G* to fetch it again as they are serving it from the cached page, so it may be 24 hrs or few days depending on the site and its importance to G*. If you disallow image hot linking, not sure but that may cause a problem for images being shown on G* images, need someone else to confirm this.
The only drawback I think is when other sites scrap your content, with relative paths, they have to manually change to the full URL otherwise they'll have a page looking like it is on Lynx browser, with this fix, it is is ready made for them, however that is not a massive problem for most sites anyway.
I implemented this today, hopefully to morrow/day after with a new cached page the blank page problem disappears from the previews feature.
I know you may say if this was the case, a css-less page should show at least, and you may be right and could well be a security precaution you implemented (mod_security for example), we are in the same boat and I'll be checking until I see it fixed.