It appears to me that there is more that unites these two authors than divides them. Fundamentally they are both interested in better support for persistent objects.
JDOs strength is that it abstracts the underlying data storage mechanism freeing the programmer from the complexity and the tedium of performing the object mapping manually. The programmer is immediately concerned with the user defined classes and a compact API to interact with the JDO PersistenceManager.
As evidenced by the reader comments, its strength is also its greatest weakness. There is at the very least a perception of a loss of control over the underlying data store. The JDO vendors tools MAY automatically generate the database schema. This schema MIGHT be non-optimal for the performance characteristics of the application or for other users of the data store. How the JDO vendor maps to the underlying data store, and the level of user involvement, is a feature of the implementation.
The strength of SQL99, as I understand it, is that it offers a potential standard (subject to its implementation by relational database vendors) for representing object models in a relatinoal database. As has already been pointed out a standard and language independant view of the object model offers several advantages when deployed in a heterogeneous environment over time. What SQL99 does not provide is a convenient API for the Java programmer.
I may be speaking out of turn, but it seem to me that these two technologies together might provide a compelling solution to the OR problem combining the strengths of both. A user friendly API that is familiar and convenient for the Java programmer and an standard schema that can be optimised by the DBA as appropriate and shared by heterogeneous clients.
The solution that I describe bears similarities to the OMG Model Driven Architecture which proposes the object model as the unit of interoperability from which specific implementations are derived, the database schema and Java classes.
Oracle have just purchased TopLink the OR Mapping Tool from WebGain. Perhaps they might lead the way by implementing such a solution.