Brian,
Thanks for you questions.
Does AON replace the Content Engine module because they also provide Application Optimization and Application Delivery functionality?
No, these modules are focused on different problems. Where the Content Engine seeks to improve the Human User to Application response profile, AON targets message traffic between applications. In other words, AON can provide accelerated handling of message traffic, such as XML processing. It is not designed to speed up the delivery of application content to users. AON can easily be used in conjunction with the Content Engine in a case like an Order Management application. The Content Engine might be used to cache a frequently accessed product catalogue closer to users of the application, while AON would be ideal for processing the XML order itself that is sent from the order application to back end order processing.
Does it replace CSS / CSM because of the load balancing capabilities?
No, Content Switching and AON load balancing work in different ways. Content Switching will load balance across sessions, and again works well in "application content to user" delivery situations. AON, on the other hand, load balances based on enforced policy. In AON, the business analyst identifies specific criteria for making load balancing decisions. For instance, AON could route messages from high value Gold customers to more performant servers, while sending messages from other customers to other servers.
Please rate this reply if I was able to answer your questions.
Terry