Form isn't sending POST parameters?
You're telling the form to submit via POST, and then once it's done submitting, to redirect to the webform. I think what you intended to do was to make the form submit its data straight to the webform, which you could do with the #action key in the $form array. See http://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/6#action

However, this isn't really a very robust approach. A better approach would be to put together your own webform, and then put that webform into a block. If you need to do any custom processing on the webform, the webform module has its own API and hooks that you can use.