Andrew from Entrust alerted us to a problem with our documentation around the Entrust Certificates that customers use with Live Communications Server 2005 SP1 and mostly with PIC. I asked Andrew to provide me with information I can share here for your consumption and he has provided this:
The majority of Entrust SSL certificate products, including Advantage SSL Certificates and UC Certificates, set the client auth attribute and so will work with the PIC feature. Only our Standard SSL Certificate product does not have the client auth attribute set.
Andrew also has a blog that he maintains. In his words from the site (Sept 30, 2008 edit, the site no longer looks to be Andrew's personal blog
http://trustme.goingon.com/ : )
<I searched my prior posts and believe that this is now the only piece of information for Entrust, if you do find something else and incorrect please alert me>