FEATURE 2012.5:

For those using the “Force Guest Checkout” it wasn’t a true “force” since there was still a link to be
able to login. Now, it truly will be forced guest checkout…

If the store setting, "Force Guest Checkout" is on or the guest parameter is used on the query string, the login info/link will be hidden on OnePageCheckout along with the password entry section and the address book dropdown. The setting is not new, but the hiding of the login info/link & address book drop down is.

Store Setting

Before the “hide” features

After this 2012.5 feature implemented.