It's true. You can generally call submit() on a form on any event, as long as you identify the form clearly. @pXL uses jQuery syntax for that; my only caution is that you need to make certain there's only one form on the page, because jQuery fires the handler on the list of elements its selector returns. Either name the form
$('form[name=theOne]')
or use an Array accessor
$('form')[0]
You can make it fire on keyUp in a field (you could test the key value to match 'return'), or even if the user clicks somewhere outside the form.
$('html').click(function(){$('form')[0].submit()});
(This is probably not the best user experience, though!)