Google offers Adsense display ads (which anyone can load up) and...

Google offers Adsense display ads (which anyone can load up) and based on what keywords you select, your ads (banners) appear on peoples websites. OpenX is different. It gives the publisher of the website control in that they control which ads go on their website. They also have an auction system which is similar I believe to what AdSlot has:
http://openx.com/publisher/real-time-bidding

There is money to be made in this market. However there is also the possibility that Google enters this market with a competing product. They already dominate Search Advertising with and Display advertising. This would be their turf. I would be worried about that. (Google coming in). Too risky for me.