Seth Godin has done some thinking about Google AdSense, and why it may be valuable to have ads for competitive products on your site:
Google AdSense ads across the top increase confidence. They’re recognizable and safe for the surfer. They make it clear that there are alternatives. They demonstrate confidence on the part of the site builder. Analogy: All car dealers do better when they’re on the same street.
Now, of course, we don't recommend that you put Google ads on your corporate site. On the other hand, if you're a software reseller, they may be appropriate and may--as Seth's experiment indicates--bolster sales.