Form theming with templates
Since you're attaching the #theme to an element, you need to user render element in your theme declaration in place of variables:

function MYMODULE_theme($existing, $type, $theme, $path) {
  return array(
    'some_form' =&gt; array(
      'template' =&gt; 'some-form',
      'render element' =&gt; 'form'
    ),
  );
}


After you clear the caches your template should start being used for that form. 

See hook_theme() for more information.

Just another thought, if your template file is as trivial as just wrapping the form in a tag you can use the #prefix and #suffix properties on the $form object instead of using a theme at all:

$form['#prefix'] = '&lt;div id="some-form"&gt;';
$form['#suffix'] = '&lt;/div&gt;';