When I send a message and the progress bar is still "moving", can I simply move along and send another message?
And even before that is completed, if I move along and send another message can I be guaranteed that all messages will be sent?
Apple's own applications have the capability to do "real" background processing and multitasking. User created applications have "some" capability (and limited time) to perform "certain" background processing and multitasking.
With that said, most apps that need to finish a task, can do so because Apple has provided a way to do such things.