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.