Aaron Wall of SEOBook did a great interview with Google engineer Matt Cutts and it can be found here.
He comments on SEO techniques being construed as possible spam:
Lots and lots of search engine optimization is white-hat and not spam at all. Things like making a site more crawlable, tweaking the words on a site based on what users type in or what you see in your server logs, and gathering links by coming up with creative ideas or services that make people link to you naturally. To me (and Google), spam is search engine optimization that is outside our quality guidelines–things like hidden text, hidden links, doorway pages filled with gibberish words that do a sneaky JavaScript redirect, and so on.
Lots of good stuff in there and makes an interesting read.