iPhone OS 4 won't help in this situation because instant messaging isn't one of the services that the os has support for background processing, it's a common misconception that OS 4 will support general multitasking, it doesn't, instead they've provided a set of background services for the most useful type of background applications, like gps, voip etc.
The combination of push and the new background services means that 99.9% of realtime situations are covered with minimal impact on battery life.
The combination of push and the new background services means that 99.9% of realtime situations are covered with minimal impact on battery life.