I've had the exact same thing on my dendrobiums and some of my catt types countless dozens of times. I've tried over and over to get an answer to what these are, and the closest thing I've come to are thrips. The little black dots are apparently their droppings of the larva (I think) and they create those white silvery patches that can't be rubbed off on the bottom of the leaves. I find that spraying with Safers End all gets rid of them.

Thanks Calvin! Strange that I can't see the adult thrips, but I will try that spray. Hey are you in Toronto, Canada? I want to visit some day.

Do you have any pictures you can post?

Here is some good photos of thrip and the damage they do. There are many species of thrip. Some are actually beneficial and eat spider mites. Unfortunately those never seem to be the ones on orchids when you need them.

Here is a link that might be useful: http://mrec.ifas.ufl.edu/lso/thripslinks.htm#DAMAGE_PHOTOS