This is what my login screen looks like when I've got both LDAP (for internal users) and Username/Password (for external users).
This is pretty confusing and bad. I've had a number of users completely derp out over the concept of using the proper log in box for their credential type.
In an ideal world, we'd have just one login prompt and be able to specify fallback rules, e.g.: try this credential as a username/password first. If that fails, try hitting LDAP. (or the other way around). A marginally more acceptable solution would be to have tabs and/or a combo box to select the credential type.