After a rash of steerer tube failures, Trek has responded blaming incompatible stems and incorrect installation. Trek advises:
1) Always use a torque wrench, and never over-tighten stem clamp bolts.
2) Always use spacers above and below the stem. Although less obvious than correct torque, a minimum of 5mm and a maximum of 40mm spacers under the steerer, plus a 5mm spacer above the stem are required. Riders should factor in these spacers when sizing their bike.
3) Use only the stem brand and model that came with the bike, because not all stems will work with carbon steerers. Often the lighter the stem, the less chance it will be compatible with a carbon steerer. Weight-relieving cutouts on the stem clamp and steerer interface can create stress risers.
You can read the full article on VeloNews. I'd rather just have a slightly heavier bike and be able to change my stem without fear of crashing.