I had an issue with a multi-page form recently where Internet Explorer would go back a page immediately upon hitting the backspace key.  If you accidentally do this while not in a text field you'll lose your form data.  Other browsers actually ask you if you want to resend data when you go back.  The solution was to disable the backspace key on my form page.  I accomplished this with the following Javascript (requires jQuery):

If you need a way to redirect webform submissions to a dynamic URL in Drupal 6, using the Webform PHP module can work well with Webform 3.x.  I recommend only using post processing conditionals on select fields if possible with Webform PHP since you need to enable the permission to "use PHP for additional processing" for users who submit webforms (typically anonymous).

Here's the process: