Okay, so I finally finished building up my first cyclocross bike (the christmas components helped finish it off) and rode it to work this morning. Another couple of firsts for me are the integrated headset and carbon fork.
I installed 4za cantis and (after much fiddling) got them tuned in. But if I apply anything more than a slight bit of pressure to the front brake the whole front end vibrates back and forth. I can look down and see the front hub shuddering front to back and handlebar assembly also shakes like crazy. My first thought was that the headset was loose, but there is no play to the steerer and -- if anything -- I thought the fork was a touch tight when I put it together.
Is this just because these are fairly powerful brakes on a so-so carbon fork? Did I screw up the headset somehow? (It came with the frame/fork set from Performance, I THINK it's an FSA, at least the top cap is.)
I'm at a bit of a loss and I'm not looking forward to bringing this in to the LBS after doing absolutely everything else (incl. wheels) myself. Any suggestions?

I have seen this happen with a few carbon forks with canti brakes.

How tightly have you done up your quick release, and what is the bearing preload on your front hub like?

First, I would wet sand the braking surface on the rims with 320 paper and file the brake pads.

Next, check that there is no play in the brake pivots. If they are even slightly loose, they can cause vibration as you brake. I would even go as far as tightening them until they just start to bind (not so they don't release, but on the verge) and testing it.

Rowan: I did tighten the QR while riding, from "sufficient" to "definitely tight enough", no change in problem. There is no play in the bearings that i can detect.
San Rensho: The rims are NOS campy tubulars (so are not machined, but anodized). Would you still recommend wetsanding? The 4zas are similar to the old Mafacs, or Paul Neo Retro (without any of the modern adjustments), so they're limited in how finely i can adjust them. Right now I've got fairly long Koolstop pads on them. Would a shorter brake pad help with the vibration given the limited adjustability re. toe-in? I'm assuming by filing you mean file to make up for this lack of trim adjustment, yes?

Sorry, I also forgot: I've always thought cantilevers (and v-brakes too, I guess) have a certain amount of unavoidable play. These 4zas have no tension screw either. (They sound less ideal the more I describe them, but they look SO good...)

Check the toe-in on the pads. that caused my cantis to shudder like you're talking about.

If you don't want to screw up the finish on the rim by sanding, just put another wheel on it and see if it makes a difference. The reason for recommending filing the pads is just to clean them up and get some good new material exposed. You can try different pads also. BTW I agree with above that it could be the toe in.

To check for brake pivot play, hold the front brake, rock the bike back and forth while holding a finger on the brake arm at the pivot to feel for play. If you have play, try using a washer/shim to get rid of it.

I have been battling the same problem on my Aurora. My fork can flex a good inch front to back and I believe it's steel. Even with installing new pads and setting the toe-in I can't get rid of it completely. I've been looking for a thread that addresses this problem.

Make sure the brake pads are turned the right way (long end towards front). The kool-stop pads shudder horribly if they are backwards.

its called fork chatter and its fairly normal on cross bikes. I wouldent bother to worry about it depending on how much chatter there is, and how often it does that. (hard brakeing will do that on most cross bikes)

I had the same problem with my cross/commuter bike. It's a Surly Crosscheck frame, Winnwood carbon cross fork and avid canti's. The first time I rode the bike it was so bad I thought I'd never be able to use it. It was worse when I was applying medium pressure on the brakes. (If I applied firm preassure it was rideable.)

I re-checked my pad adjustment (I seem to remember setting a pretty generous 'toe-in' at first), sanded the pads, and after a few days break-in things improved. I eventually switched to V-brakes. Honestly, I don't know why they still make canti's, but I digress. Once in a while the fork will still shudder when I'm just barely dragging the front brake alone. But don't worry. Your bike will be fine.

DanO