Additional note, June, 2009:

It has been suggested that the requirement that a static data member be initialized in the class definition is not actually needed but that static data members should be treated like other variable declarations -- a preceding definition with initialization should be sufficient. That is, given

    extern const int i;
    const int i = 5;
    struct S {
      static const int j;
    };
    const int S::j = 5;
    int a1[i];
    int a2[S::j];

there doesn't appear to be a good rationale for making a1 well-formed and a2 ill-formed. Some major implementations accept the declaration of a2 without error.