Working around a lack of element queries: http://filamentgroup.com/lab/element_query_workarounds/

_widget.html.erb

<!-- this partial takes a boolean "respond" argument -->

<div class="<%= respond ? "widget--respond" : "widget" %>">
  ...
</div>

_widget.sass

.widget
  color: skinny

.widget--respond
  @extend .widget

  +at-breakpoint(32.5em)
    color: fat
 
// Using Susy's at-breakpoint mixin for brevity. This outputs:
//
// .widget, .widget--respond {
//   color: skinny;
// }
//
// @media (min-width: 32.5em) {
//   .widget--respond {
//     color: fat;
//   }
// }

homepage.html.erb

<div class="content">...</div>
<aside>
  <%= render "widget", :locals => { :respond => false } %>
</aside>

schedule.html.erb

<div class="content">
  <%= render "widget", :locals => { :respond => true } %>
</div>