The code below showed a login form for rails 2.3.8 thru 3.0.10 but with the latest upgrade to rails 3.1.8 the form no longer displays (but the rest of the page does).
Possibly an issue with the form_tag?
%h3 Login
- form_tag do
%fieldset
%legend Please Log In
%table
%tr
%td
%label{:for => "name"} Name:
%td= text_field_tag :username, params[:username]
%tr
%td
%label{:for => "password"} Password:
%td
= password_field_tag :password, params[:password]
%tr
%td{:colspan => "2"}
= submit_tag "Login"
Result:

Fairly simple one for this – change
to
Now it shows!