Just to make this concrete, here's a small program demonstrating the
problem.  Compile and run it, first against 1.4.2, then against
1.5/5.0:

import java.sql.Timestamp;
import java.util.Date;

public class Foo {
  public static void main(String[] args) {
    Date d = new Date();
    Timestamp ts = new Timestamp(d.getTime());
    System.out.println(ts.compareTo(d));
  }
}

Note that this a *binary compatibility problem*!!  The overload
resolution is the same in 1.4.2 and 1.5.0;  in either case, it's
compareTo(Date).  With a 1.4.2 runtime, it dispatches to the version
defined in java.util.Date, with a 1.5.0 runtime it dispatches to the
version defined in java.sql.Timestamp, which promptly casts its
argument to Timestamp (and blows up).  This really feels like a P2 bug
to me.