try something like this:
rhtml: <%= text_field_with_auto_complete :person, :nachname, {}, { :select => 'name', :after_update_element => "function(element,value) " + "{ " + remote_function(:update=>'Kundendaten', :url=>{:action=>:update_kundendaten}, :with=>"'adr_id='+value.id") + ";" + "}" } %>
controller: def update_kundendaten #params[:adr_id] should be ok in controller end
but with this solution i have the problem that the autocomplete is only
working on IE.
FF wont show the autocomplete popup. 
HTH