CiviCRM+Drupal not sending Confirmation Email upon User Registration

Behavior: When a new user registers on the Drupal/CiviCRM web site through the standard user registration interface, the web site fails to send out any confirmation email and simply redirects to the original home page without any further information. The lack of any email makes it very difficult for new users to work out what they are meant to do after registering. However, the user can go through the "Forgot Password?" link to get the activation email.

Expected Behavior: Upon registration, the web site should send out a confirmation email with a link to activate the user's account.

Notes: I realize that this issue is related to Drupal's registration process, but it appears that the CiviCRM installation (more particularly the CiviProfile registration form) is causing this problem:

Steps to replicate (copied from a forum post - it also applies to CiviCRM 3.1.2):

1.- Install Drupal 6.15
2.- Configure Drupal for sending an email to the new users with the information for login (verify the email address)
3.- Register as a new user. -> Drupal send an email to the user telling: your login is..blah... and your pass is DJFHDDKSj
4.- The user can login without problems.
5.- Install CiviCRM 3.1.1 and configure the basics...or not. Even without touching the config doesn't work.
6.- Try to make the same. New user -> The user is created in Drupal _and_ on CiviCRM. Everything is OK...just...the user doesn't receive any email and cannot login because he doesn't have the password!
7.- If you say: I forgot the password, please resend me on...that works! Drupal send the email with the new activation.

For more info, see the forum thread: http://forum.civicrm.org/index.php?topic=11999.0