Apple does not currently allow 3rd party iPhone applications to run as background processes. Instead, apps must shut down completely after use. Push notifications was Apple's solution to allow applications to receive notices while they are not the active application. Certain services, such as Instant Messaging, would benefit greatly from this feature.
Certain services..... how about all services. Until Apple build a 'real' mobile OS that allows me the intelligence to choose any and all apps I want running at any one time.
Oh and i also want to be given the choice of 'what' apps i install rather than those deemed acceptable by the apple censors.