Route::controller(
    'customers' , 'customersController',
    array(
        'getIndex'  =>'customers.index',
        'postUpdate'=>'customers.update'
    )
);

<?php
class customersController extends BaseController
{
    public function getIndex()
    {
        if ( Auth::check() ){
            $customers = new Customers;
            return View::make('layouts.customers')->with('customers', $customers);
        }
        return Redirect::route('dashboard');
    }
    public function postUpdate($id)
    {
        print_r( $id);
        die;
    }
}

?>

{{ Form::model($customers,array('route' => array('customers.update', $customers->id))) }}
...
{{ Form::submit('UPDATE',
                                array('class'=>'btn btn-default btn-default-small') ) }}
{{ Form::close() }}

Missing argument 1 for customersController::postUpdate()