You could also use a two-pronged approach:
<%= … select box code :preferences, :job_history_filter_period %>
…
<%= observe_field 'preferences[job_history_filter_period], :update => ‘some field’, :url => {:action => :modify_history_time_period} %>
Though I do notice that you shouldn’t need the :with in the first place. The Rails helpers should automatically send back params[:job_history_filter_period] through remote_function, though I may be wrong.
Jason