Just because you see failed aluminum frames on the 'net doesn't mean that aluminum is significantly more prone to failure. How do the sales numbers stack up between aluminum and steel bikes nowadays? If aluminum outsells steel and has comparable failure rates, wouldn't it make sense that we'd see more failed aluminum frames?

Engineering is an art of compromises and any material can fail depending on which compromises are made. Want a bike that's impervious to frame failure? Sure, but it'll probably be a bit on the heavy side, whether it's made from aluminum or steel. Want a bike that's lightweight? Sure, we can do that, in a number of materials. But it'll probably be a bit more prone to failure since we'll push the envelope a little bit to get it juuuust that much lighter. Want that bike to be stiff, too? And without being too expensive? No wonder aluminum's so popular.

OK, but only since you asked so nicely...

Aside from the absolute newest composite designs, most modern airframes are built primarily from aluminum. A Boeing 747-400 is made of 74 tons of aluminum. The Boeing 777 is 50% aluminum. The structure of the brand-new Airbus A380, the largest airliner in the world, is 61% aluminum. 80% of an F-16 fighter's structure is aluminum. B-52 bombers are primarily aluminum and have an estimated service life of 37,500 flight hours -- about 4.25 years in the sky -- before metal fatigue becomes a concern.