XMLGregorianCalendarImpl class uses some methods that are only availalble in Tiger, and thus fail to load on earlier JVMs.
JAXP 1.3 is supposed to be able to run with JDK 1.3, so this is a spec violation.

For one example, see the following method where it uses "new BigDecimal(long)":

    public void setMillisecond(int millisecond) {
if (millisecond == DatatypeConstants.FIELD_UNDEFINED) {
fractionalSecond = null;
} else {
checkFieldValueConstraint(MILLISECOND, millisecond);
fractionalSecond = new BigDecimal((long) millisecond).movePointLeft(3);
}
    }

A bigger issue is the fact that this went unnoticed. JAXP should be built with JDK 1.3 (so that javac can report compile-time errors on cases like this), and it should be tested with JDK 1.3 also.