You can't issue a getMBeanInfo call from a client running JDK 5 to a server running JDK 6 or 7. A CORBA registry (tnameserv) is used server side.
If the client runs JDK 6 or 7 all goes well.
If both server and client run JDK 5 all goes well too.
Used 5 U17 b05, 6 U12 b02 and 7 b41.
Before the getMBeanInfo we connect and do a createMBean that goes well.
When the client does the getMBeanInfo it gets rejected and prints the following when debug is on:

FINEST: Calling the method getDefaultDomain.
Dec 11, 2008 12:26:56 PM com.sun.corba.se.impl.transport.SocketOrChannelConnectionImpl <init>
WARNING: "IOP00410201: (COMM_FAILURE) Connection failure: socketType: IIOP_CLEAR_TEXT; hostname: 129.157.209.96; port: 42886"
org.omg.CORBA.COMM_FAILURE: vmcid: SUN minor code: 201 completed: No
at com.sun.corba.se.impl.logging.ORBUtilSystemException.connectFailure(ORBUtilSystemException.java:2172)
at com.sun.corba.se.impl.logging.ORBUtilSystemException.connectFailure(ORBUtilSystemException.java:2193)
at com.sun.corba.se.impl.transport.SocketOrChannelConnectionImpl.<init>(SocketOrChannelConnectionImpl.java:205)
at com.sun.corba.se.impl.transport.SocketOrChannelConnectionImpl.<init>(SocketOrChannelConnectionImpl.java:218)
at com.sun.corba.se.impl.transport.SocketOrChannelContactInfoImpl.createConnection(SocketOrChannelContactInfoImpl.java:101)
at com.sun.corba.se.impl.protocol.CorbaClientRequestDispatcherImpl.beginRequest(CorbaClientRequestDispatcherImpl.java:152)
at com.sun.corba.se.impl.protocol.CorbaClientDelegateImpl.request(CorbaClientDelegateImpl.java:118)
at org.omg.CORBA.portable.ObjectImpl._request(ObjectImpl.java:431)
at org.omg.stub.javax.management.remote.rmi._RMIConnection_Stub.getDefaultDomain(Unknown Source)
at javax.management.remote.rmi.RMIConnector$RMIClientCommunicatorAdmin.gotIOException(RMIConnector.java:1420)
at javax.management.remote.rmi.RMIConnector$RemoteMBeanServerConnection.getMBeanInfo(RMIConnector.java:1033)
at javasoft.sqe.jmx.test.functional.ORBMBeanInfoTest.run(ORBMBeanInfoTest.java:69)
at javasoft.sqe.jmx.share.TestStarter.<init>(TestStarter.java:28)
at javasoft.sqe.jmx.share.TestStarter.main(TestStarter.java:79)
Caused by: java.net.ConnectException: Connection refused
at sun.nio.ch.Net.connect(Native Method)
at sun.nio.ch.SocketChannelImpl.connect(SocketChannelImpl.java:464)
at java.nio.channels.SocketChannel.open(SocketChannel.java:146)
at com.sun.corba.se.impl.transport.DefaultSocketFactoryImpl.createSocket(DefaultSocketFactoryImpl.java:60)
at com.sun.corba.se.impl.transport.SocketOrChannelConnectionImpl.<init>(SocketOrChannelConnectionImpl.java:188)
... 11 more
Dec 11, 2008 12:26:56 PM RMIConnector close

Still with debug output we see the server receives the getMBeanInfo but no answer seem to be sent back:

Dec 11, 2008 12:25:06 PM RMIConnectionImpl getDefaultDomain
FINEST: connectionId=iiop: 2
Dec 11, 2008 12:25:06 PM ServerCommunicatorAdmin reqIncoming
FINER: Receive a new request.
Dec 11, 2008 12:25:06 PM ServerCommunicatorAdmin reqIncoming
FINER: Finish a request.
Dec 11, 2008 12:25:06 PM ServerCommunicatorAdmin synchro
FINER: Admin: Timestamp=1228994706582
Dec 11, 2008 12:25:06 PM ServerCommunicatorAdmin synchro
FINER: Admin: elapsed=1
Dec 11, 2008 12:25:06 PM ServerCommunicatorAdmin synchro
FINER: Admin: remaining timeout=119999
Dec 11, 2008 12:25:06 PM ServerCommunicatorAdmin Timeout-run
FINER: Waiting with timeout: 119999 ms remaining

Attached full test output (look for server.out, server.err, client.out, client.err).