MyndFyre provided the insight that attenuation is exponential in distance traveled. This would mean the effects of attenuation become exponentially (c^n) stronger as distance increases.
No, it's (on average; assuming no ferrous metals are interfering) proportional to the square of the distance -- flux through a shell does not change based on radius, so the power density decreases just as the surface area of a shell increases.
This isn't a subject I find particularly interesting, and I don't claim to be well versed in it.
From a quick glance of the wikipedia article, though, it seems that attenuation can be exponential.