A High Rankings Forum member has an interesting question about a website he maintains. In the particular example, he is selling a niche item within a broader category. I'll use an example for illustrative purposes: he's basically selling "children's books." However, he notices that he's getting general traffic from "books" sites, though his specific focus is children's books. Now he realizes that he could potentially benefit from the "books" site traffic, so he's coded some different content to be served based on a particular search phrase. If you're searching for "books" in general, you'll get different content than if you're searching for "children's books," he explains.
Is this a problem?
High Rankings forum member rolf says it might be. It feels like a win-win situation -- you're serving the right content for your visitors while still benefiting from the traffic. But is it safe? It's hard to say. As rolf puts it, "you are technically breaking the rule about serving one thing to spiders and another to visitors."
Forum discussion continues at High Rankings.