The admin/login.html template includes an empty 'breadcrumbs' block, to make sure that any breadcrumbs are suppressed. As the login form is presented in a small frame, much like a popup where breadcrumbs are also supressed, this seems reasonable.
However, while nav-global blocks are suppressed in popups (along with the entire header), they do however appear on the login dialog. This causes the login page to look strange, at the least, for anyone employing the nav-global block in a custom admin/base_site.html or admin/base.html.
Inserting {% block nav-global %}{% endblock %} near the empty breadcrumbs block would eliminate this eyesore.
I am doing so in a custom admin/login.html currently to deal with this, but I think this is probably an oversight in the 'real' admin/login.html. Understandable in that the nav-global block is probably rarely used.
1.2 is feature-frozen, moving this feature request off the milestone.