My new dendrobium victoria reginae started losing leaves. I can think of two possible reasons why: either at 90+F my apartment is too hot for this cool-grower to handle, or its mount was drying out too much. I can't do anything about the heat until my air conditioner arrives in the mail, but I did reconsider my original decision to mount the plant.
I know that this species likes lots of water, and in this heat, the mount was drying up within hours. While my tolumnias have been thriving under these conditions, the den victoria reginae probably needs a more even amount of moisture.
When I took the dendrobium off its mount, the roots seemed in good condition. I kept the sphagnum moss around the roots, and potted the whole thing into a 4inch clear plastic pot. I filled out the rest of the pot with oncidium potting mix. Depending on how the roots fare in this mix, I may decided to switch later to small bark pieces, or something else like that.
Edit (11/16/12): This orchid died not long after I wrote this post. It lost all its leaves and the canes shriveled up. I'm guessing that it could not tolerate the unairconditioned heat of NYC summer. Since this is my second failed attempt with Den. victoriae-reginae, I have now given up on trying to grow this species.