I don't want to mislead anyone here. There are MANY things that can lead to a vibration in the steering wheel. If your doing all the normal checks, I would add the test that vettestr recommended and do a quick visual on the tire as it's spinning on the car. I eliminated one possible cause at a time which lead me to tires. I just wish I started there first!
In my case I "believe" the out of roundness of the tire manifested itself as a harmonic vibration at 65 MPH and 75 MPH. One harmonic was the front tires and one was the back tires transmitted though the frame. It may be a bit worse in the fronts for FFRs as they seem to have inherent bump steer problem.