Error in the form_alter hook due to PHP 5.3 on Ubuntu 10.04 LTS

If we create a content of this type & add in it a field with "Number of values" set to "Unlimited," when you click on the "Add another item" button, you get the following warning:

warning: Parameter 2 to content_profile_registration_form_alter() expected to be a reference, value given in /[your var/www/ folder path]/includes/common.inc on line 2883.

This is only happening on PHP 5.3 on Ubuntu 10.04 LTS. This bug goes away on PHP 5.2.9.