Bicycle Mechanics - High speed wobble/shimmy

I was on a ride today going downhill. Occassionally, I applied both the back and front brakes to slow. At one point the bike began shaking violently. I thought first I was going to crash and I thought the cause might be a flat. I braked slowly and just prior to stopping the wobble went away. I checked my max speed and it was around 42 MPH.

I was in the drops at the time. I've gone faster without a problem. The road was fairly new chip seal. This was the first ride I've done since moving the seat back. I have ultegra brakes. I didn't notice any wobble in the tire, just the whole bike shaking. It did seem to be the front tire.

Any ideas what might have caused this?

You hit the right speed in the right conditions that caused the bike to vibrate. You can either speed up, slow down, and/or press your knees against the top tube to reduce/remove the vibration.

side effect of light, flexible, frames .. Dynamic balancing is done to Motor bike wheels
not bike wheels ,
ready to have an absolutely perfect frame/ fork alignment job done?
[only really practical cold setting steel.. ]
Or, You might just want to keep your speed down , in the future ..

Any ideas what might have caused this?

At a guess, moving the seat back means you now have less weight on the handlebars, which creates less damping in the steering. Read on:
http://www.sheldonbrown.com/brandt/shimmy.html

Any ideas what might have caused this? I'd start looking for a loose headset. Next, I'd look for loose hub bearings. Third, and least likely, I'd check the spoke tension.

One way to minimize shimmy while coasting, is to put one's knees together.

As unreasonably doubtful as this may sound, check the rear wheel first. Also, check for any suspicious frame anomalies... again, focusing on everything from the seatpost and back.