Behaviour of threads when entering background in iOS multitasking

While running some tests I've noticed that when an app (with no background privileges, i.e. voip, gps, etc) enters the background state, any background threads (or concurrent NSOperations) running at the time seem to be paused (not terminated), and they continue when the app enters the foreground.

I can't see this thread behaviour documented anywhere. Does anyone know if this behaviour is defined and can be counted on?

It would be useful to not have to cancel tasks or get involved in task completion if the thread can safely continue when the app is re-opened. Especially for complicated operations.

Any info would be great. Thanks!