index.html.erb:

Filters:
<% form_tag '', :id => 'filter_form' do %>
  <%= select (nil, "project_id", Project.find(:all).collect {|p| [ p.name, p.id ] },
  {:prompt => "Select a Project"}) %>
  <%= select (nil, "sprint_id", Sprint.find(:all).collect {|s| [ s.scheduled, s.id ] },
  {:prompt => "Select a Sprint"}) %>
  <%= select (nil, "user_id", User.find(:all).collect {|u| [ u.name, u.id ] },
  {:prompt => "Select a User"}) %>
<% end %>
<%= observe_form :filter_form,
                  :frequency => 0.5,
                  :update => 'display',
                  :url => {:action=>'filter'},
                  :with => [ :project_id, :sprint_id, :user_id ] %>

<div id='display'>
</div>

INSIDE THE CONTROLLER:

def filter
    @tasks = Task.find( :all, :conditions => { :project_id => params[:project_id],
                                               :sprint_id => params[:sprint_id] } )
    render :update do |page|
      page.replace_html 'display', :partial => 'display'
    end
  end