select_tag question

How can I use select_tag to

1. Have a prompt like "Select a Value" ( selected by default )

I tried this but did not work:

  <%= select_tag :current_course,
                 options_for_select( Course.find(:all).map {|p|
[p.title,p.id]} ),
         { :prompt => "Select a Value" }
    %>

2. Generate a 'selected' event so that some action can be done like
submit the form or refresh the page?

Thanks in advance
- AJ