Good question and good answers.
Perhaps we should point out the reason that Apple does this is for the stability, reliability and security of the device. "Multitasking" on any computing device demands more resources from the hardware which is no big deal on today's powerful computers, but on such a small computing device with more limited resources like the iPhone which runs on a battery that could be problematic. The more processes and threads running simultaneously the more likely the device is to perform poorly, have apps crash, and the battery will run out of power sooner. Over the years our group has tested numerous devices like Pocket PC/Windows mobile which allow you to multitask and the battery life is a joke. Simply put, restricting what can run increases the stability and performance of what IS running on a device and increases battery life. Rememb that the thing also runs wireless communications for voice and data all the time (amazing what we have in small devices these days) those are major tasks.
Apple has thus far chosen this route so that things are simpler, run better, and last longer. That being said you can still listen to your music or talk on the phone while doing other things on the phone. Say, look up a map while talking. Others like Palm Pre and Android allow unlimited multitasking which is fine and suits some people better, but I don't really care about doing so much at one time on a small device. I think it is a bit silly to be honest, I mean how much can you do on a 3" screen? It is a legitimate requirement for some to be sure, but I think you can make a pretty good case that the restriction is a feature and not a flaw.
Everyone else is copying the iPhone in Apple's wake and that complaint just doesn't hold water for me. It is an amazing device and they absolutely smoked every one else with it, even Blackberry.
Missing Copy/Paste was a huge hole in the iPhone, now THAT was a true flaw.
