I used to have a Stewart microperf which dropped down in front of a Fujitsu 50" plasma and I could absolutely see a reflection--a very bright one. The screen material was about 4 inches in front of the plasma glass. I ended up having to make a dark cloth cover that I placed over the plasma whenever I used the drop down screen, which was a pain. A black backing would probably take care of this problem, although I don't know of any manufacturers that have this solution (maybe Screen Research??).
The visibility of the reflection depends on the angles between the projector, the plasma, and your eye. Unfortunately, for most people the projector is mounted high, the plasma is on the middle of the wall, and your eyes are low, so it's like a headlight shining off a mirror into your face. If the projector was mounted low in a cabinet or coffee table, you might not have a problem.