Planes are usually designed to specific tolerances and requirements. Every bolt and rivet has a specific purpose. If it wasn't needed, it wouldn't be there (to save cost and weight).
[link|http://www.69megs.com/pages/whenmagazine/Stealth97/stealth.htm|Here] is a story about a crash of an F-117 stealth fighter/bomber in 1997. It crashed because some bolts were left out during maintenance, or were misinstalled.
The AA crash yesterday seems strange to me. I can understand an engine falling off and the pilot losing control. One would expect that if a engine was going to be lost that it would happen at takeoff when the stress is greatest. But there are reports that both engines came off. It seems unlikely that both would be lost (from opposite sides of the plane). And why did the tail fin come off?
Jet engines are tested to make sure they survive eating birds. (There have been threads here about it.) Even if both engines injested birds, why would the tail have been lost? They'd only been airborne a few minutes and were presumably still accelerating to cruising speed. The stress on the tail fin shouldn't have been that great even if the plane was out of control - I would think....
There was a case in 1996 when an [link|http://www.cnn.com/US/9607/06/delta.plane.2/|engine failed] and a piece of the turbine penetrated the cabin and killed a passenger and her child. It was early enough in the takeoff that the pilot was able to abort. So it's not unique to have an engine come apart even though they're very reliable and very strong (the blades are made of titanium and there's a whole technology built up around them - e.g. [link|http://www.google.com/search?q=cache:1cRXzqSjOiI:www.ml.afrl.af.mil/successes/2000/ss00413.html+titanium+fan+blades&hl=en&start=4|this] article from Google's cache.).
The AA case strikes me as a very strange accident based on what's reported now.
But I'm no expert on this stuff.
Cheers,
Scott.