What are the reasons for using a modal window? What is it doing for you or your users?
Is this a home page with normal home page content where a user has access to a button or link called "sign in" and instead of going to a new page, gets a lightbox-like login?
I suppose one possible benefit of a modal is, unlike a real link taking users to a dedicated login page, we are saving an HTTP request. Like a dedicated page, the user is presented the form without other page clutter in the way. No distractions. However it seems most pages who need logins get logged into without using either separate dedicated pages or modals.
I won't say more until you describe what you've got and why a bit more. Cheers.