Q:

Why is 'estar muerto' used instead of 'ser muerto'?

I know it is rather rude to think of it this way and I don't want to offend anyone religiously, but being dead is usually thought of as a very permanent condition in the United States.
So why does Spanish use the word 'estar' instead of 'ser' to indicate that condition? Is there some background to this in terms of religion or politeness?
If I say 'Mi padre es muerto', would that have a drastically different meaning than if I had used estar, or is the first one just plain incorrect and should never be used at all?

A:

Although it is true that estar usually indicates a non-permanent, temporary condition whereas ser usually signals a permanent condition, you have to take into account that estar is normally used to express a condition resulting from a transformation, process or actual change, and ser is normally used when referring to an inherent characteristic, with no involvement of process, change or transformation; in other words, ser normally has the purpose of including the subject into a certain class.
This explains why there are some adjectives that express a permanent condition but can only be combined with estar: this is so because they express the result of a change or transformation; muerto is one of those adjectives, and another example is roto: el vaso está roto and not el vaso es roto.

A:

Because estar is used to signify that they weren't always dead--that they were once alive.
Ser is used to indicate a state of being--that they were always that way and always will be.
To say es muerto is incorrect because they used to be alive.  Therefore, estar.

A:

While agreeing with Gonzalo Medina's explanation, I would like to add my rule of thumb for this one. "Ser" means that the property does not change for the object, while "Estar" means a transition. 
So, "él está muerto" because he was alive before, but "mi camiseta es verde" because it will not change by itself(*1). In the case of "verde", you can also say "la manzana está aún verde", because while the T-shirt does not change by itself, the apple does.

