As the other posters said, PB Blaster is the stuff for rusted/frozen nuts and bolts. It penetrates the rust seal and is in the threads faster than anything else I've used. Another product that works well is Kroil. Liquid Wrench works too, but for my money, PB Blaster is the best.

The uses for WD-40 are for primarily displacing water (for instance, inside the distributor cap and on the plug wires of a water-splashed engine), secondarily as a light oil for protecting metals from corrosion, keeping drill bits and saw blades cool while in use, as a lubricant while tapping threads, and in a pinch it will loosen decals enough to allow their removal. So it has valid uses. It just doesn't work especially well as a penetrant.