Shan McArthur blogged about why you would want to consider using Live ID authentication on your customer portal that is integrated with CRM. Shan gives an overview of how Live ID works, and focuses in on how to make it work in an ASP.NET application using standard forms authentication and a membership provider. The article also covers how you associate your web users authenticated with Live ID with records in your CRM.
Shan's blog article
http://www.shanmcarthur.net/crm/articles/crm-customer-portal-user-authentication-with-live-id