Validate that the username is alphanumeric and is not already
        in use. Don't fail if users username is provided.

def clean_username(self):
        """
        Validate that the username is alphanumeric and is not already
        in use. Don't fail if users username is provided.
        """
        user = None
        try:
            user = User.objects.get(username__iexact=self.\
                    cleaned_data['username'])
        except User.DoesNotExist:
            return self.cleaned_data['username']

        if user:
            if user.username == self.user.username:
                return self.cleaned_data['username']

        raise forms.ValidationError(_(\
                "A user with that username already exists."))