I have a Canon 430EX flash and an iPhone 5. I really enjoy the quality of the camera in daylight, but of course in low light conditions the built in LED flash is quite poor. So would it be possible to use an off camera flash such as the one I have to provide additional light for the camera? I would prefer to use this in practice and not just for theoretical purposes.
You can use an optical trigger app in your iPhone. This reverses the roles, the iPhone takes a picture when you fire your flash.
The one app I know that does this is iSyncFlash.
I know with the 430ex II I can set it to manual and wireless and the flash from the camera will trigger the speedlite. Just give it a shot.
Here's a video where one of the cheapest cameras was used with a 430ex II: http://www.youtube.com/watch?v=zh6zr3wKRV0
The iSyncFlash app is supposed to do this, by syncing the iPhone's camera to the flash, rather than the other way around (although the current reviews of the app indicates it doesn't work much on phones newer than a 4s). And you may be able to accomplish this with TriggerTrap Mobile, two Android/iOS devices, and a flash adapter; TriggerTrap Mobile uses the headphone jack for the sync signal.
3 years ago I bought a nova flash for iPhone and have been using it ever since with great success. it fits in a credit card slot in my wallet and works with the Nova app. You can also use more then 1 if you wish to have multiple flashes. Great option for extra light and the app is very good with editing tools and presets. Battery last a very long time and it turns on via BT as soon as you open the app. There is no switch on the flash itself. there are different setting for the typ of light and intensity. I love my Nova.