I'm redacting an email I sent to vortex-l and putting it up here, since the list appears to be down.
In a recent thread on vortex-l, Axil mentioned that tungsten has a low hydrogen permeability and that this causes problems for Brillouin's and Widom and Larsen's hypotheses. He provided some interesting links, and he appears to be correct about the permeability of hydrogen-1 in tungsten.
I find the low hydrogen-1 permeability an encouraging result, for roundabout reasons. A conjecture that I think we should consider is that ionization of the hydrogen isotope (hydrogen-1, hydrogen-2, etc.) is a requirement for a cold fusion reaction to proceed. One of the questions that has been bugging me is why hydrogen-1 appears to work well with nickel but palladium seems to require deuterium, and hydrogen-1, if anything, seems to interfere. This could be an overstatement; there have been many experiments, and I wouldn't be surprised if there is some countervailing evidence, but this seems to be the general trend of what is being seen, as far as I can tell.
The low hydrogen-1 permeability of tungsten lends credence to the notion that the size of the lattice (and, apparently, it's Miller number -- 100, 110, etc.) is a factor here. So we might guess that palladium allows for the diffusion of monoatomic hydrogen, but it does not yield high levels of hydrogen ions, whereas nickel perhaps does. The following link is suggestive concerning the migration of monoatomic hydrogen (rather than unshielded protons) in palladium:
Like others, I think the heat-after-death effect, where a reaction continues after the current has been stopped, is not central to what is going on, so I see no strict need to require diffusion of hydrogen in the bulk of the cathode. Indeed, there is evidence that what is going on is a surface or near-surface reaction; the low permeability of hydrogen in tungsten seems to point in this direction as well. There is the question of the need for high loading in Pd/D electrolytic systems to see an effect; one possible explanation here is that you just need a high enough concentration of free deuterons on (or near) the surface of the cathode to see results in a situation in which we barely control the reaction, and high loading gives rise to this as a side effect, due to the desorption over time of large numbers of deuterons.
Ionization is also something that would happen in the glow discharge and electric arc experiments.
What role might ionization play? To pursue my pet hypothesis, perhaps you need an unshielded proton or deuteron in order for something to happen in connection with the its electrostatic charge. If bulk loading only plays an indirect role in specific systems and it was not needed in previous tungsten experiments because, for example, sufficient ionization was brought about through other means, it's not clear what the implications are for Brillouin's, Widom and Larsen's, or for that matter, Peter Hagelstein's hypotheses.