Orchids - Orchid bark only?

It sounds as if you've received a Phaleonopsis orchid. The guy from the garden center gave you sound advise. You don't need to transplant it for quite some time, because they like to be root-bound a bit. As long as they have a good drainage source, they will last for a long time. If it seems to be wilting, the blooms themselves are probably coming to the end of their blossoming period. The blooms will need a period of dormancy before they re-bloom for you. It will love it in your humid, bright bathroom! :) I work for a florist, and orchids are the one plant that panic the most people...