The battery impact of the OS handling multitasking for 3rd party apps in addition to system apps is negligible in terms of processor power usage. As correctly pointed out already, memory is more of an issue to the system for multitasking. However, battery life is impacted by 3rd party background apps that use the network, GPS, and other hardware. Restricting the types of background processing and even how they are implemented is for power management reasons.
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As for compatibility, if Apple were to introduce an unrestricted service object like Android has, all current applications that might take advantage of background processing would require a redesign to use a client/service architecture. Developers would have to maintain a completely different app for pre-iPhone OS 4 devices or iPhone OS 4 devices with no multitasking. In addition, the backend server infrastructure required for PNS would only be used for pre-iPhone OS 4 devices and non-multitasking iPhone OS 4. This would fragment development severely.
[via David Quintana]
Una chiara spiegazione di come il multitasking delle applicazioni sia diverso in iPhone OS 4.0 e Android senza entrare troppo nel tecnico. La scelta restrittiva di Apple è quella per la quale propendo.
