Microsoft's update to the My Xbox Live app 1.5 brought along support for controlling an Xbox 360 straight from an iPhone. (The update was rolled out across all iOS platforms, but for whatever reason, this feature only works with an iPhone.) Here's how to get it to work.
First, you'll want to make sure that your Xbox 360 and iPhone are connected to the same Wi-Fi network. You can check the Wi-Fi status by going to settings on the iPhone and network settings on the Xbox. Once you've confirmed that both devices are using the same Wi-Fi, head over to the settings area again on the Xbox and then enter Console Settings. From there, you'll want to navigate down to Xbox Companion. Click on Available, then return to the home screen. Your Xbox is now ready to interact with any kind of companion application.