Robin Jewsbury has an useful post on mobile advertising on the Forum Nokia Blog. He describes his experiences of using mobile advertising and why he uses more than one advert provider to improve income.
Robin mentions the main problem with mobile advertising – scaling. Whether you are using adverts, providing them or in some mobile operations, trying to balance adverts and advertisers, maintaining ad inventory can be tricky. I have worked with some ad-based companies that have struggled to maintain this balance.
While I know some ad networks deal in adverts from other networks, many people are taking the do-it-yourself approach to try to get the best of balancing advertising supply, demand and income. I think there could be opportunities in this area for consolidation in libraries, APIs or new services.