Not for a static data member of class type, as in your example.
9.4.2/2:
The declaration of a static data
member in its class definition is not
a definition ... The definition for
a static data member shall appear in a
namespace scope enclosing the member’s
class definition.
9.4.2/4:
If a static data member is of const
integral or const enumeration type,
its declaration in the class
definition can specify a
constant-initializer which shall be an
integral constant expression (5.19).
In that case, the member can appear in
integral constant expressions. The
member shall still be defined in a
name- space scope if it is used in the
program and the namespace scope
definition shall not contain an
initializer.