I've always heard of people doing this but I never ventured to try it myself. The thought of putting a piece of wood on a grill and expecting not to have a fire escaped me. I took the plunge, however, and tried it this weekend. I have to say that I was quite pleased with what transpired.
I took a cedar plank (a Christmas present from 2 years ago) and soaked it for about 6 hours. The house smelled like a sauna all afternoon. My sink wasn't big enough to allow me to submerge the entire piece of wood, so I flipped the board every hour or so. (Note to self: don't do that again. Make sure the whole thing is wet the whole time.) Next, I made a marinade for the salmon. I discovered, after the fact, that it really didn't need it, but it didn't hurt it. I saved part of the marinade to use as sauce. I mixed about 1/4 cup dijon mustard, a few teaspoons of olive oil, a few teaspoons of white wine, and a little salt in a small bowl and brushed some on the salmon. The salmon sat out while the grill was prepared.
When the grill got hot, the cedar plank went on the grates. After three minutes on one side (and a lot of wood popping), I flipped it and placed the salmon on the plank. Then the grill cover was shut, and the grill did the rest of the work. The salmon basically smoked/steamed for about 13 minutes. I removed it, and let it sit on the plank for another 2 minutes, then the salmon went on a plate and in my belly.
The great thing about cooking salmon on a cedar plank is that fish stays moist and soaks up a little of the cedar flavor. Just remember that the fish continues to cook on the plank after you've removed it from the grill, so don't leave it on too long or you'll accidentally overcook salmon (an unforgivable act in our household.)