Associate webform submissions with the user created I am allowing anonymous user create submissions, and creating a user programmatically based on these submissions. How do I associate this submission with the user I've just created?

UPDATE: I thought I could use db_update for this but I'd be interested to know if there is a more functional way.
I would recommend checking out the available hooks provided by the Webform module: http://cgit.drupalcode.org/webform/tree/webform.api.php

It looks like hook_webform_submission_insert() will be your best bet, with that you should be able to not only programmatically create the user at that point, but then update the submission with the newly created user id.

function my_module_webform_submission_insert($node, $submission) {
  // Create new user.
  // Retrieve new user id.
  // Update webform submission with new user id.
}