something very similar happened to a couple of my vegetable plants, it turned out to be a nutrient deficiency but i don't know how likely it is that this is the case here.
also dig around in the mix, see if there're some buggies hiding there... do orchids tolerate neem oil? a good preventative spray couldn't hurt, its what i do with the veggies...
i feel your pain, i've had spots more like what joeW described as bacterial or fungal and all i can do is remove infected parts and hope more don't show up too quickly. kinda makes me feel helpless...