Like he said the input in the armrest is just an audio input. Plug anything signal from any device into it and it will play, you do however have to control the device from the device controls.
If you have an ipod and want integrated controls (i.e. stereo buttons control the ipod) then you should go with one of the many ipod integration methods available. I wouldn't suggest the OEM one as it's far less functional than most of the other ones. Also you don't have to get the $113 switchbox to use the ipod adapter. Here's what the switchbox does. Since both the ipod adapter and the ACI use the same plug on the HU it allows both to plug in at the same time, and then you access the functions by pushing the buttons on the stereo face. If you don't plan on plugging anything into the port then there's no point in getting the switchbox. Also most all of the aftermarket ipod adapters have integrated 3.5mm inputs, so if you went with one of them you would still have another input. And you could still get the switchbox and have two inputs if you really needed them.