Newbie Question: Are arguments in a function definition bound when function is called?

Well, no need to get upset;   Stavros'  explanation said it all, in some 
sense.

You can't cover over the functional meaning of a built-in function by a 
value binding.

g(sin):=sin(sin).

g(%pi/2)  ---> 1

It is not  (%pi/2)  viewed as a function
applied to the argument (%pi/2) as an argument.

It is sin, the built-in function,   applied to the value %pi/2

Actually, the function name need not be built-in, but any function.

Inside the body of  g, above,   the name sin   is used as a function and 
as a value.  They are different.

If you want to use the VALUE of a name as a function,  try apply(f,[x]).

Let's try

h(sin):=  apply(sin,[sin]);

h(f) -->   f(f)

h(%pi/2)  -->   error, improper name or value in functional position, %pi/2


I hope this helps.