Many newer, digital-only lenses (e.g., Tamron's Di II series) as well as many newer standard 35mm lenses (e.g., Tamron's Di series) have coatings on the back of the rear lens element. The intent is to reduce the chance of light bouncing off the image sensor onto the rear lens element and causing flare or otherwise distorting or degrading the image. The thought behind it being that the sensor, microlenses, IR cut filter, etc. are fairly highly reflective.

Does it make a difference? Probably not in most situations especially if your shooting technique is sound and you already take steps to reduce the chances of flare in your images.

That's what I've already read. I'm wondering if it's just something designed to boost sales of third party vendors because Nikon and (I assume) Canon don't say anything about it.

Some Nikon-mount lenses were next to useless on the full-frame Kodak 14n for exactly this reason. The real element was reflective and so was the sensor, setting up a loop of reflections. The results I saw ranged from almost no contrast to multiple ghost images from light sources in the picture area.

Sensors want the light entering as nearly perpendicular as possible, and lenses designed for digital do this, including the Nikon DX line. Leica with its M8 actually has angled the sensor elements on the outer fringes of the sensor, inward to allow the use of true - not retrofocus - wide angle lenses. I believe that the firmware also has settings that help compensate.

It would not surprise me to hear that at least Nikon and Canon lenses originally made for film, but manufactured in the past few years received anti-reflection coatings, but I have no information that this is true.

There are two factors regarding the possibility that a reflection from the sensor surface would get reflected back on the sensor and cause a secondary exposure problem. One is that most lens rear elements have considerable curvature, although I've not seen every lens and some may be close to flat, but that seems a rarity. With a curved rear lens element any light shining on it will be dispersed widely and little back onto the sensor. The second factor is the fact that many film emulsions surfaces were both light in color and very shiny, but this fact did not contribute to any such problems.

The reason I suspect this is not much of a real problem is light reflected from a sensor surface, or film for that matter, will be reflected by the rear element of a lens very little, the main reason being that most of the light will pass through the element into the interior of the lens and be absorbed by the dark matte surfaces of the interior barrel of the lens.