user name and passphrase dialogs accept non-ASCII characters

Dialogs which accept user names or passwords from the user should not allow anything else than ASCII for those fields. Currently it is possible to configure a virtual XKB layout in the X session and even enable a input method framework, like IBus, to enter arbitrary Unicode characters in these fields. This can leave the user stranded in several situations:
 * GDM does not have any input method framework enabled, the input method framework only starts within the X session, i.e. _after_ login.
 * remote logins when done from computers which don't have the necessary characters in the configured keyboard layout or the necessary input method available
 * shell logins on the console, in case X has crashed or so. By default the keyboard layout on the console cannot be switched unless you have logged into your session.
ASCII is the set of characters which always work on any keyboard by default.

This issue affects all programs which set user names and passwords and those where the user needs to type the password to unlock a service.

=== original bug report ===

Binary package hint: ibus

After upgrade to Ubuntu 9.10RC, I'm testing the IBus input method.

Then I found that IBus can be activated in the passphrase dialog such as gksudo and
screensaver, although passphrases can contain only alphabetical or numerical characters.
This behavior might make the beginners confused.

I think IBus should be disabled in such dialogs.