I've used modal forms to pop-in a template containing the login form and some XHTML, but I've found it very frustrating when not using the provided forms.
I ended up injecting the provided login form into my template.
I'm wondering how to use this module properly if you want to load neither a simple form, neither simple XHTML, but a form and XHTML together.
For my first try, I simply rendered the default drupal login form and I saw it was rewritten by modal forms in order to work properly in ajax.
But, once submitted, logging in occured but without correct redirection, thus making the modal window crash, trying to refresh itself.
Any information would be appreciated.