Due to webform using a pattern in its hook_theme form webform_form key, it is not possible to add template suggestions in hook_preprocess_webform_form.
First issue is that the pattern does not follow conventions i.e.
The convention is to use __ to differentiate the dynamic portion of the theme. For example, to allow forums to be themed individually, the pattern might be: 'forum__'. Then, when the forum is themed, call:
theme(array('forum__' . $tid, 'forum'), $forum)
Second issue is that if a pattern is specified, it will always be taken into account as per http://drupalcode.org/project/drupal.git/blob/1d4604da252f0e6e19339957ec... so it would be best to delete the pattern and work with template suggestions (as node.module does http://api.drupal.org/api/drupal/modules%21node%21node.module/function/t...)
The problem with that change is that all templates must be renamed from webform-form-1.tpl.php to webform-form--1.tpl.php to agree with the convention.
Quick patch coming soon