I asked in the Google forums a while ago along these same lines when I was writing my article about the benefits of reporting spam. The answer I got was one that does encourage a better, brighter Web world for everyone (lol). But at what cost?

As mentioned above, of course the hidden text is underhanded, and it does indeed fall under the category of spam (more specifically, in this case, 'keyword stuffing', being that we're talking about content that is not deceptive, just hidden). It may only be a little bit of spam, but it's enough. And it can be awfully frustrating to even the most steadfast of us knowing that others are out there breaking all the rules when we're here left holding the empty bag on what seems the lonely, impotent 'good side' of the online content guidelines.

But there is recourse. Offending sites can be reported to search engines, and the sites can be penalized or even banned from SERPs. I know this sounds nuts, but it's nuts only if you're disapproving of the idea that one or two people good enough to report spamdexing can do damage to a site in violation. And you'd be right! But it's times like these where our principle of dog-eat-dog capitalism really bites us in the rear. We assume that only one or two people would report a site, and so we throw up our hand disgusted and beaten. There's no need to fret, however. All anyone needs to do to get the job done is to convince other site owners invested in the same target audience to ally themselves to clean up their respective fields of interest by reporting spam sites together.

But who's really going to go through all that trouble of contacting site owners, establishing a relationship, and coordinating a mass reporting of identified spam sites, right? Exactly. And that's why nothing ever gets done. Only the worst of the worst get hammered, because they're obvious. The rest? They're lucky if they catch an evil eye.

Here's a few resources that may help:
-report spam on Yahoo
-report spam on Google
-report spam on MSN
-report spam through SEOTools
-report spam on Ask Jeeves or Teoma

Good luck,
The Lion.