Index: django/core/validators.py
===================================================================
--- django/core/validators.py	(revision 7104)
+++ django/core/validators.py	(working copy)
@@ -23,6 +23,7 @@
 _datere = r'\d{4}-\d{1,2}-\d{1,2}'
 _timere = r'(?:[01]?[0-9]|2[0-3]):[0-5][0-9](?::[0-5][0-9])?'
 alnum_re = re.compile(r'^\w+$')
+alnumdot_re = re.compile(r'^[\.\w]+')
 alnumurl_re = re.compile(r'^[-\w/]+$')
 ansi_date_re = re.compile('^%s$' % _datere)
 ansi_time_re = re.compile('^%s$' % _timere)
@@ -67,6 +68,10 @@
     def __str__(self):
         return str(self.messages)
 
+def isAlphaDotNumeric(field_data, all_data):
+    if not alnumdot_re.search(field_data):
+        raise ValidationError, "This value must contain only letters, dots, numbers and underscores."
+
 def isAlphaNumeric(field_data, all_data):
     if not alnum_re.search(field_data):
         raise ValidationError, _("This value must contain only letters, numbers and underscores.")
Index: django/contrib/auth/models.py
===================================================================
--- django/contrib/auth/models.py	(revision 7104)
+++ django/contrib/auth/models.py	(working copy)
@@ -128,7 +128,7 @@
 
     Username and password are required. Other fields are optional.
     """
-    username = models.CharField(_('username'), max_length=30, unique=True, validator_list=[validators.isAlphaNumeric], help_text=_("Required. 30 characters or fewer. Alphanumeric characters only (letters, digits and underscores)."))
+    username = models.CharField(_('username'), max_length=30, unique=True, validator_list=[validators.isAlphaDotNumeric], help_text=_("Required. 30 characters or fewer. Alphanumeric characters only (letters, digits and underscores)."))
     first_name = models.CharField(_('first name'), max_length=30, blank=True)
     last_name = models.CharField(_('last name'), max_length=30, blank=True)
     email = models.EmailField(_('e-mail address'), blank=True)
Index: django/contrib/auth/forms.py
===================================================================
--- django/contrib/auth/forms.py	(revision 7104)
+++ django/contrib/auth/forms.py	(working copy)
@@ -11,7 +11,7 @@
     def __init__(self):
         self.fields = (
             oldforms.TextField(field_name='username', length=30, max_length=30, is_required=True,
-                validator_list=[validators.isAlphaNumeric, self.isValidUsername]),
+                validator_list=[validators.isAlphaDotNumeric, self.isValidUsername]),
             oldforms.PasswordField(field_name='password1', length=30, max_length=60, is_required=True),
             oldforms.PasswordField(field_name='password2', length=30, max_length=60, is_required=True,
                 validator_list=[validators.AlwaysMatchesOtherField('password1', _("The two password fields didn't match."))]),