Okay, so I want to use the twitter bootstrap modal to display the devise sign-in form.
I am basing my code of the wiki article here: https://github.com/plataformatec
This is the code that ended up working
<%= simple_form_for(:user, :url => user_session_path) do |f| %>
Don't use :data => { :dismiss => "modal"}. That should trigger some js function from bootstrap that overrides the form submitting event
:data => { :dismiss => "modal"}