I wasn't sure where to post this, and I am pretty sure others have/had this problem but here goes:
I got an '06 SS ce 2.4l and when braking at highway speeds the steering wheel shakes very violently. I do believe some of this has to do with a warped rotor, however the brakes are all new as of 5,000 miles ago. I however feel it is something related to the steering column. If i drive on like a dirt road, or go over some bumps in the road, I can hear this sort of knocking coming from the fire wall area, and if I am sitting still, I rock the steering wheel side to side quickly and can make that very noise appear. My guess is the u-joint is going out, and therefore there is some play there before it catches one side or the other. Like i said though, I am sure others have this problem and if someone could just direct me to the cause, and whether its worth the expense fixing, I would greatly appreciate it! I also feel quite a bit of vibration in the wheel, mostly at highway speeds as well-compared to my girlfriends cobalt which has no vibration whatsoever, and does not make the noise either.

 

have you hit your wheels on a curb hard? Does the car pull a certain way when trying to drive straight? You may need a camber adjustment or just a plain wheel alignment..I would also check your front end components also when its getting the adjustment. As for the wheel shaking at high speeds when braking, your rotors (like you said) are probably warped, and uneven brake pad wear can cause this, which is caused by making hard quick stops

EDIT: now, dont quote me on this, but IMO these are the symptoms of these problems. just my .02. goodluck!

 

I have the same problem on my 08 TC. I'm getting a new steering column under warranty. The sad thing is I only have 8k miles on the car...