Aluminum has been around in the bike business for many years. My first "serious" road bike was an '87 Cannondale R400 equipped with some extremely unreliable Suntour Alpha 1000 components a Sugino crankset and Dia Compe brakes (simple design, but they worked fine).
This monster of a frame, which I just donated to Bikes-Not-Bombs in Roxbury, Mass., was built from welded 6000 series Aluminum. The BB shell was massive, the chainstays were massive and it sprinted well, but every pebble in the road felt like you just hit a railroad track. I won't bore you with a lot of metalurgical hooey, but aluminum has a considerably higher strength to weight ration than steel.
So, Cannondale (and Klein and Vitus -- early aluminum frame-makers) could use more material in the frames to prevented power-robbing flex without paying a weight penalty. Aluminum however, is far more brittle. You can't bend it back and forth a million times like steel without breaking it.
Carbon fiber is a bonded material using well..fibers and epoxy. Anyone that's ever worked with the stuff knows it's tough. Why Trek has chosen aluminum steerers over all carbon forks remains a mystery to me. Think about it, the thinnest part of the fork is on the legs and those are carbon right?
Both of my road bikes have all carbon steerers and I am built more like an ex-NFL fullback than a cyclist. For Hincapie, the constant pounding of riding Cobbles at 26 mph is bound to lead to fatigue and failure as we witnessed today by Big George. He didn't help matters by using a long 140 mm stem which acts like a lever, nor by his of choice using deep rim wheels over the more common Roubaix choice of standard rims and 32 spokes.
Hindsight being 20/20, I'm not ready to dump on his mechanic, he used the best products available by his team. George also had basically no stack height above his headset cups either, which might have helped. Photos of his bike after the crash show that the steerer snapped in the middle not the crown or at the stem. The fatal flaw was George's training ride where he went out to punish the wheels to see if carbon wheels could handle the cobbles. Carbon 1, Aluminum 0.
If you are currently running a carbon fork with an aluminum steerer (Reynolds Ouzo Comp, Cannondale Slice or anything made by Trek), my advice would be to pop it out and check for stress fractures. Stems and bars fail too, so look for gouges from an over zealous wrench. If you have ever crashed the front of your bike I would give some serious thought to replacing the fork, stem and bar. George is a pro, he knew how to hit the dirt (nice roll). You and I might not be so lucky.