I have a client who just asked for this (rate based upon mileage distance) as well. In their case, they will be shipping furniture and using a freight shipping company. So in short, there appears to be multiple scenarios (though perhaps not common) for determining shipping by mileage.
I understand that this would require a plugin. I am a developer, and it is likely I could develop this feature (given enough time and pay from the client). If I do want to try developing such a plugin, do you have any recommended plugin for using its code as a starting point?