There appears to be a contradiction in Structures as to whether a complexType 
with simpleContent is allowed to be derived by restriction from a mixed type.

The property mapping rules for complex type with simple content state the 
following for content type when restriction is chosen:

"1 if the type definition resolved to by the actual value of the base 
[attribute] is a complex type definition (whose own {content type} must be a 
simple type definition, see below) and the restriction alternative is 
chosen ...".

In addition, Schema Representation Constraint: Complex Type Definition 
Representation OK states:

"If the <simpleContent> alternative is chosen, the type definition resolved to 
by the actual value of the base [attribute] must be either a complex type 
definition whose {content type} is a simple type definition or, only if the 
<extension> alternative is also chosen, a simple type definition; "

However, "Schema Component Constraint: Derivation Valid (Restriction, Complex)" 
states:

"5.1 If the {content type} of the complex type definition is a simple type 
definition, then one of the following must be true:

5.1.1 The {content type} of the {base type definition} must be a simple type 
definition of which the {content type} is a valid restriction as defined in 
Derivation Valid (Restriction, Simple).

5.1.2 The {base type definition} must be mixed and have a particle which is 
emptiable as defined in Particle Emptiable). "

See: http://lists.w3.org/Archives/Member/w3c-xml-schema-ig/2001Jun/0047.html