PG, Thanks for posting these. You might also look at some of the ptarmigan nutrition work done by West and Meng in the 60s at UAF (try Google Scholar). While they primarily focus on diet, it is not a big stretch to guess that habitat partitioning is driven by diet. Robert Moss also had a very interesting paper, 'The digestion and intake of winter foods by wild ptarmigan in Alaska,' in which he describes the energy budgets of all three ptarmigan species. Willows appear to have much higher energetic costs than the other two species, possibly due to their feeding ecology - they feed on woodier twigs moreso than the other species, and tend to be in more exposed habitats while feeding. I guess it might have some effect on how the three species overlap.
After thinking a bit more about mixed flocks of ptarmigan, I think that only during migration (September/October and March/April) have I seen rocks and willows together. This makes sense, as they are highly vulnerable to predation at this time, and more birds together decreases the chance of any one bird getting nailed by a predator. Where I have seen birds in close proximity during the winter months, there were small groups close together of separate species, but I would not go so far as to call them truly mixed flocks.
Jim, as to your comment on seeing birds budding on aspen - I have only observed this in willows, and sometimes in really odd places (downtown Fairbanks, for example). Aspen is very nutritious, but is difficult to digest (I'm stretching here from some studies in moose), so maybe it is a trade-off between nutrition and digestibility.
If it ever gets back into the reasonable zone (above zero), I'll have to get the dog out and do some more 'sampling' to support my theories... I'll be sure to report my findings to the group...