Well, I don't see any platform restrictions in the text of the classpath exception here: http://openjdk.java.net/legal/gplv2+ce.html

I know they require that an alternative implementation have platform restrictions before they'll let it run the TCK (which was the problem with Apache Harmony), but I don't see any in the license for OpenJDK itself. Is there some other source I should be looking at?

Full text, from there (applying only to source files that specifically say "subject to the 'Classpath' exception" in the header):

    Linking this library statically or dynamically with other modules is making
    a combined work based on this library.  Thus, the terms and conditions of
    the GNU General Public License cover the whole combination.

    As a special exception, the copyright holders of this library give you
    permission to link this library with independent modules to produce an
    executable, regardless of the license terms of these independent modules,
    and to copy and distribute the resulting executable under terms of your
    choice, provided that you also meet, for each linked independent module,
    the terms and conditions of the license of that module.  An independent
    module is a module which is not derived from or based on this library.  If
    you modify this library, you may extend this exception to your version of
    the library, but you are not obligated to do so.  If you do not wish to do
    so, delete this exception statement from your version.