# Somewhere in your application:

from registration.forms import RegistrationForm
from django.forms import ValidationError
from django.core.urlresolvers import resolve, Resolver404
from urlparse import urlparse

class NewRegistrationForm(RegistrationForm):

    # Ensures than any usernames added will not
    # conflict with existing commands.

 def clean_username(self):
        username = super(NewRegistrationForm, self).clean_username()
        try:    resolve(urlparse('/' + username + '/')[2])
        except Resolver404, e:
            return username

        raise ValidationError(
            (u'This username does not create '
             u'a valid URL.  Please choose '
             u'another'))


# in urls.py:

url(r'^accounts/register/$',
    'registration.views.register',
    { 'form_class': NewRegistrationForm },
    name = 'registration_register')