Steve,
If you're still around, I just wanted to say "Thanks!" for conducting this for everyone to see. I just now found this thread while trying to find more infomation about the little white, fluffy knots that show up on cuttings.
With my Sal (Gene's) in particular, I have hordes of these all over the wood, and they wipe away easily, and show a bump in its place. At first I thought this was a root initial, so I experimented with two of the cuttings, potted one and left the other one in a baggie. Neither one of them rooted from those points after several weeks.
I suppose this is not a root initial, so ... I also wonder what these starchy, fluffy prominences are?