I've read that a standard 'flood and drain' hydroponic method is all very well for orchids, but if you get a disease it can transfer to them ALL, through the water.
That's why what most people use here is a technique devised by Ray called Semi-Hydroponic. Take a look here at his info on the subject.
Semi-hydroponics This is the way a lot of people here grow their orchids and I've got about 1/4 of mine growing well this way.
Also we have a sub-section of the forum for Semi-Hydroponic (S/H) here.
Semi-Hydroponic Culture - Orchid Board - Most Complete Orchid Forum on the web !