Hijack, sorta:
I'm getting a wobble in the
front wheel (
Shimano WH-R540 (pdf)) when I take my hands off the bars at descent speed. Hub and headset are both solid. Even with the wheel wobbling there's no appreciable vibration at the bars. I'm thinking there's a resonance between the rim/tire and the axle.
Flip your front wheel and see if the problem persists. Sometimes, a tire can cause this.