More than likely the discs are perfectly true, but uneven brake pad deposits cause the vibration you feel especially if it is mild and most noticeable at speed. I have had some problems with uneven brake pad deposits on my stoptech rotors recently and having previously beleived the masses that my discs might be warped, I thought the only solution was machining, skimming or rotor replacement. In fact this is not the case, as a moderate and fairly new vibration can be cured by fitting agressive and abrasive pads, performing a few quick stops using the aggressive pads cold, and not allowing them to heat up and transfer more pad material. Basically this info is all on the stoptech website, but having bought some Hawk 9020 racing pads and tried their advice, the brakes are prefectly smooth without hint of the previous vibrations.
This explains also a problem with the brakes on my previous car. Having used them without proper bed in procedures and then quite harshly on the road, the car developed a vibration. A track session with some red stuffs fitted smoothed out the vibration completely....
Buying a set of "scrub pads" is a cheaper option than continuously skimming or replacing the disc, however, careful bed in and attention paid to cool down seems to avoid further issues.