You remember it too! I googled and found the text pasted on this blog: http://thegoldenwrench.blogspot.com/201 ... rding.html
...and because I had a hard time finding it, here is the text:
A tech bulletin from Trek regarding carbon assembly paste
Trek has recently learned that carbon paste (of any brand) can damage a carbon steerer.
With most parts, correct clamping prevents any relative motion between the parts. For example, the stem firmly clamps the handlebar and the frame firmly clamps the seatpost; there is no movement. In these cases, carbon paste performs the valuable function of reducing the amount of torque required to affect a solid attachment of the parts.
However, with a headset there is movement of the parts. In normal usage of a threadless headset there is some slight movement of the compression ring relative to the steerer. This movement occurs when you hit a bump or apply the front brake. Normally, the movement of the compression ring does not hurt the fork. However, if there is carbon paste between the compression ring and a carbon steerer, the paste works like an abrasive to remove carbon material from the steerer. If enough material is removed, this could lead to breakage of the steerer.
What Trek is doing
The Trek Owner’s Manual and Assembly instructions say this about assembling carbon parts:
The carbon part can be assembled in a clean, dry state. As an alternative, we recommend the following special carbon prep products:
Tacx carbon assembly compound
Fiber Grip™ carbon fi ber assembly gel from Finish Line
All Trek publications will be amended to say that carbon paste should never be applied to the steerer.
What you should do
Stop applying carbon paste to carbon steerers. Although it might seem possible to only apply paste under the stem where there is no movement of the clamp, we have seen that paste can migrate down the steerer. This might happen on its own, or through exchanging parts or positions of existing parts. In any case, we recommend avoiding any contact between carbon paste and any part of a carbon steerer. Instead, clean both the steerer and the compression ring with rubbing alcohol and a shop rag; clean until the rag shows no discoloration.
Educate your customers. If a customer brings in a bicycle with a carbon steerer, inspect the steerer and if necessary, clean it. Also please explain this information to the customer. Trek is committed to the safety of our riders, so if you find a steerer with significant scoring, please contact your Rep for a free replacement fork. You will be asked to follow normal procedures, and the jpeg image you submit must be reviewed by Trek before any further action can be taken.