You are going to need something to record sound for your iPhone film or for your low budget digital feature film. There are a number of ways to go about this.
First you could use native audio recording. In other words use the mic that is part of your iPhone or Dslr or Camcorder. Forget about this idea. It will not work. These devises are not designed to record quality audio. At least not the quality that you are looking for.
The thing that will scream amateur quicker than anything is poor sound quality. The audience will forgive poor picture quality (sometimes this is a choice by the filmmaker) but they will not forgive poor sound.
There is a way around this. You will have to shoot separate audio and sync it later. Not as scary as it sounds, that is how most movies are made.
For the Dslr cameras I would recommend getting a good mic and attaching it to a separate audio recording device. The Zoom h1 or 2 or 4 and or getting a juicelink audio mixer and using it with a good mic to record audio. Here is a tutorial to help you.
Next up I would like to talk about prosumer camcorders and the best sound solutions for them. If you are on a micro budget I would suggest getting a solid mic like the Rode and mounted onto the camcorder and use it to enhance the sound recording ability of the camcorder. There have been impressive results using this technique.
Although if you can afford it I would suggest plugging the mic into a audio mixer rather than straight into the camera.
Here are a few tutorials.