At the recent Web 2.0 expo (which I totally wish I went to), Matt Cutts presented on "what Google knows about spam." I actually watched the 9+ minute video (yay me!) and found it informative. Basically, he summarizes a few key points: you should build reputation and trust into your system, you can thwart and frustrate spammers (as they're people), and you should try to make your system hard to infiltrate by spammers.
He even added a slide (so that he wouldn't be "burned in effigy," as he put it), to acknowledge that SEO is not spam and that if you work with whitehat SEOs, they'll be pretty open in terms of what they're doing to optimize your site.
Here's the video, or you can go to Matt's site to watch the video, see the slides, and read the tons of comments he received.
Thanks Matt for the useful presentation. You did a good job. :)