Got a new set of wheels recently . They are carbon clinchers, 38mm, with aero bladed spokes. I was riding down hill on a long ride this weekend. At just above 40 mph, my wheel started to wobble, worse and worse with each rotation. It wasn’t loose within the fork, the fork and the wheel were swaying back and forth, with longer and longer sways. I was fortunate enough to slow down the bike and get it under control before anything horrific happened (although I was already planning my bailout strategy in my head). I checked the bike after, no brake rub, not loose within the fork, no obvious signs of anything wrong. I was fine the rest of the way, even on a long descent, though I kept my speed at or under 30mph the whole time.
Any clue as to what happened?
http://www.sheldonbrown.com/brandt/shimmy.html
this is one of those unpredictable things that can happen to bikes where nothing in particular is wrong, but an unfortunate combination of circumstances.
might have a shop make sure the wheels are well trued just in case though.
Speed wobble. Terrifying the first time you experience it. This is an excellent read and might help answer your question(s). My initial thoughts are a loose hub, but as a mechanic I would never assume this without seeing the bike/wheels first. Much more information is needed here! Get it in to your trusted mechanic, explain your concerns, and if he takes care of you present a complimentary six-pack 
Sounds like a classic case of crosswind instability. With deep section wheels wind hitting the wheel at just the right yaw angle can set up an oscillation that can feel terrifying. This seems to only happen when circumstances are “just right”. I have been on 50mm wheels for the past two years and that very thing has happened to me twice, once on a training ride and once in a road race causing me to ride right off the road. Most of the time it is not an issue but when it happens you need to bring the speed down as quickly as possible because it seems that once the shimmy begins it steadily gets worse regardless of what I do and slowing is the only thing to do. This is not speed dependent either, I just got back from a windy circuit race with descents hitting 40mph and no stability issues. Good luck with the wheels and always be on your guard.
Not to bust in here but ~38mil is not really deep dish. For speed wobble, you have to look at the system as a whole. You can say that crosswind could be *one *factor, but you can’t rule out many other factors as well. With speed wobble, a LOT needs to be taken into account. Harmonics are more about stabilizing the system versus just preparing for the worst on every ride :]
Yeah, my first thought was loose headset, but everything was tight. Upon further thought, I think it was probably just a cross-wind grabbing the wheel and my force against it that made it worse. It was probably exacerbated by just the right conditions as I had just come out of a wooded section that was protecting me from the cross wind until the bottom of the hill.
Very scary and will probably contact manufacturer or lbs.