Here's how I do it:
In my views directory I created a directory called shared, where I put any partials that are used in more than one place.
In there I have a partial named _quick_login_form.rhtml that creates the form fields.
Then in my layout I have this:
<div class = "quick-login">
<% form_tag :action => 'login', :controller => :login do %>
<%= render :partial => '/shared/quick_login_form' %>
<%= submit_tag "Member Login" %><br>
<span id="very-small-link"><%= link_to "Forgot your Login?", :action => :forgotten_login, :controller => :login %></span>
<% end %>
You could put the whole form in the partial, of course.