Select Multiple - does it work? Help!

I realize that doing a select with multiple must work, yet despite so
many attempts following dozens of examples I have yet to get any
success. I have the following (example) code:

<%= select_tag 'test[]', options_for_select(["one", "two", "three"],
["one"]), { :multiple => true, :size =>5, :id => "test" } %>

Note I have the brackets after test - so Rails knows its an array.
When I select one and two in the form I get this:

"test"=>["two"]

in my log, instead of both entries. What am I doing wrong? This
should be a simple task and I've been arguing with it all day.

Here is the syntax for multiple-select:

    <%= select_tag 'categories[]', 
        options_for_select( @categories ), { :multiple => true,
                                             :size => 15,
                                             :style  => "width:250px" }
    %>

It looks like you don’t need the :id parameter. If you select multiple values, an array with multiple ids will be submitted to the server.

Unfortunately when I do what you've got verbatim - I get one parameter
(whatever is the last selected one) sent, as an array.