Exchange 2010 and Office Communications Server 2007 R2 have a fantastic integration which enables presence and IM functionality through Exchange 2010's Outlook Web App. Once configured, this really makes Outlook Web App much
more compelling! That said, there is one very important (and not well- documented) requirement of the certificate bound to the Exchange 2010 client access server hosting Outlook Web App - the common name of this certificate must equal the FQDN of the client access server name.
For example, if my Exchange CAS server's hostname is cahub01 and my AD domain is contoso.local, the common name for the certificate should be "cahub01.contoso.local". In the Subject Alernative Names for the certificate I would add autodiscover.contoso.local and owa.contoso.local, among others.
"What if I've already purchased and installed a certificate?", you ask? Well, fret not - check out this
blog post [specifically, the "Outlook Web App Trusted Services Entry Must Match the Simple DNS Namespace" section] regarding AD modifications that will make things work for you. Enjoy!