Alot of people have iphones and a lot of people break them so it would seem logical to be able to fix them.
And a lot of the people that break them have insurance or just won't bother, so it might not be as lucrative a plan as you might think.
You also need to take practical considerations into account. Will you offer some guarantee over the repair? If I send my broken iphone off and have it repaired for £50, I'm going to want some form of guarantee it won't break the next day. If it does and you can't re-repair it, what are you going to do? The traditional "big company" approach would be to pass on a new iphone and swallow the comparatively small loss. As a small company, I doubt this is something you can really do...