It's not clear from the spec what to do in this case. Bullet 5.2.2.2 of 
constraint [1] deals with default element values. 5.2.2.2.1 talks about complex 
types with mixed content, and 5.2.2.2.2 talks about complex types with simple 
type content. So there is no rule for elements with simple types.

Maybe 5.2.2.2.2 should be changed to say if the actual type is a simple type or 
if it's a complex type with simple type content?

[1] http://www.w3.org/TR/xmlschema-1/#cvc-elt

See the first issue raised in :
http://lists.w3.org/Archives/Member/w3c-xml-schema-ig/2002Nov/0234.html 

Also see a response:
http://lists.w3.org/Archives/Public/www-xml-schema-
comments/2002OctDec/0056.html