We believe that the width of the Mill (on the sorts of family members one would expect to use for sim) makes a software implementation practical for low-grade RNGs like a LFSR. As mentioned, we expect to use an i/o like interface for a crypto box on those members that have one. The purpose of the hardware random seed generator that I mentioned is not for use as a RNG – not enough entropy even for low grade – but to provide a physics-based starting point that cannot be predicted even if an attacker has all the code and the machine circuits to study.
Whether the seed generator uses oscillators, back-biased diodes, or the howling of cats is up to the hardware guys and likely will vary by member. 
However, all this projection is very premature, and will no doubt be changed as we get closer to actual market.