Display: none considered spam?

Here’s what I was just reading over at Matt Cutts’ Blog, in an article about SEO Mistakes:

I don�t recommend that people use CSS to hide text, and I don�t recommend that they document it, either.

So what does that mean? Is it really not clever to use common CSS hiding methods? That would mean that one had to eliminate all “display: none“, “visibility:hidden“, and “text-indent: -xxxxx“, etc. And it would somehow mean that Google does actually interpret all CSS code on your website. I dunno if that is a good thing. That way, it’s possible that Google drops good resources out of the SERPs. Just think of all the blogs that use CSS. Most of them use “display: none” or something like that, too. If Google really thinks they can fight spam by looking for thing hidden via CSS, they have to have a very smart algorithm. One that ensures that sites with i.e. image-replacement are not considered irrelevant.

In a comment, Matt says that if you�re straight-out using CSS to hide text, don�t be surprised if that is called spam. Yes, okay. But… How will Google know? Where is the difference if you are an automated search-robot?