I've not yet been able to get a video to play on the iPhone, and I'm assuming it has something to do with the if statement encapsulating the PlayMovie command. My iPhoneStreamingAssets folder is next to the Assets folder in the main file structure.
Hello
to answer your question, first I must ask what version of unity iphone are you using? is it pro or is it basic.
because it seems that there is nothing wrong with your code so I assume that your unity iphone is the basic version which unfortunately doesn't allow you to play video.
I hope that helped