One of the new features of 11g is case-sensitive password.
It causes problem for Toad because Toad makes the password captial silently.
If you are using lower case password, you will get
ORA-01017: invalid username/password; logon denied.
How to disable this new featuer:
alter system set sec_case_sensitive_logon=FALSE;
A new column PASSWORD_VERSIONS has been added in dba_users may be related to this.
Not quite sure on this.
The values (10G 11G) remine same before/after changing the system parameter sec_case_sensitive_logon.