Salmon on the grill. One of my favorite things. Cook 'em over a medium heat. They should still be pink when they're done. The middle will be pink as well and still tender. Overcook salmon and it will be tough and chewy and not very tasty. Many places serve it medium-rare to prevent this problem, but I prefer mine a little closer to medium. 20 minutes or so ought to be about right. I turn them every 5 minutes or so to keep them cooking even, particularly if they're thick.
Another tip you can do with meats when grilling is to coat them in a little bit of olive oil before you put them on the grill. This will keep the natural juices in a bit better and it will also prevent carcinogens from the open flame from accumulating which improves the flavor (IMHO) and is also healthier since carcinogens are a known proponent of cancerous cells.
jag