If you need to execute some long running task on the background in Unity, you may find this tool I just wrote, useful. This is BackgroundWorker for Unity 3D
BackGround Worker for Unity3D
You will only need the class BackgroundWorker
It worth noting that a lot of seemingly background tasks can be implemented with Unity Coroutines. For example using WWW service to send or receive data from a remote server. Or any case where we just wait for a response from a remote computer.