I am able to get the issue you describe in both 6.5 and 6.6 when I connect using a 9i client.
However if I connect using a 10g or 11g client it works properly.
Is it possible that you have more then one oracle client installed and 6.5 was using 10 or 11g client and now in 6.6 it using the default client which happens to be 9i?
Please select to create a new oracle connection and in the logon dialog check what you have selected in the oracle_hone/client field.
If 9i client is selected, drop the list and select a 10 or 11g client and check the “set as default sql navigator home” option ON.
That was it, because of business objects we have our oracle default home set to 9. I changed navigator to default to the 10. Seems the upgrade went back to my default oracle home. I have set Navigator to always use the 10.