title: Equivalent of GCD serial dispatch queue in iOS 3.x
body: <p>Apple's Grand Central Dispatch (GCD) is great, but only works on iOS 4.0 or greater. Apple's <a href="http://developer.apple.com/library/mac/#documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationObjects/OperationObjects.html#//apple_ref/doc/uid/TP40008091-CH101-SW1">documentation</a> says, "[A] serialized operation queue does not offer quite the same behavior as a serial dispatch queue in Grand Central Dispatch does" (because the queue is not FIFO, but order is determined by dependencies and priorities).</p>
  
  <p>What is the right way to achieve the same effect as GCD's serial dispatch queues while supporting OS versions before GCD was released? Or put another way, what is the recommended way to handle simple background processing (doing web service requests, etc.) in iOS apps that want to support versions less than 4.0?</p>