Wikipedia holds the answers...http://en.wikipedia.org/wiki/Aluminumhttp://en.wikipedia.org/wiki/Fatigue_limit
All it really means is that
1) Aluminium can corrode, but once it starts, the corrosion usually protects the material underneath (as long as it is not in contact with a different metal)
2) More care must be taken with aluminum not to put too much stress on the material; this is why aluminum frames are thicker than steel ones
Aluminium is also much harder to weld. And welding adds a point where both stress and corrosion are more likely.
I would say that aluminium frames may not be appropriate for some applications. Thousands of miles of heavily loaded touring is one of those applications.
However, aluminium is fine for most cycling applications. I have an aluminium bike which I use for commuting and occasional utility use or light touring. For most of last year, it was my primary means of transport.
The design of the frame should have accounted for years / thousands fo miles of riding, and with a wide range of weights, surfaces, etc., so unless it is abused, it will last for years / thousands of miles. The only precaution I would recommend under normal usage is to occasionally check the frame for cracks.
I expect my aluminium bicycle to last for as many years as I want to ride it.
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