Also, double-check that your Focus Area Switch on the back of the camera is set to "Single-Area" (top most if I recall correctly). Use the pad-controller to select the center sensor in the viewfinder since it's the most sensitive out of all the array. Overlay on top of a good contrasty target with strong edges and half-press.
And don't laugh - also double-check that your lens is set to AF. You won't believe how many people get caught on that. For some lenses, it is very very easy to move that wafer switch to MF inadvertently.