I promised earlier in Using LCS 2005 with multiple domains that I would get back to you with a list of Commercial Certification Authorities that supports Subject Alternate/Alternative Names (a.k.a SAN or SubjectAltNames). These types of certificates are required for making autoconfiguration of Office Communicator work in Enterprise deployments of LCS (Using DNS and SRV records). For more info on autoconfiguration look at Shawn Mahans webcast and the article mentioned in the start. SAN's are also required for supporting Autodiscovery in Exchange 2007 (e.g. one certificate serving both mail.inceptio.dk and autodiscovery.inceptio.dk). Find a bit more information on this subject on the Exchange Ninjas Wiki.
These are my findings -
Verisign - VeriSign has a MPKI (Managed PKI) SSL offering, that includes the ability to issue certificates with multiple domain names. Its part of their Enterprise offering though, so you will need to contact their sales for help on getting an enterprise account before being able to order it.
GlobalSign - When I checked last week "... yet to be launched; expected timescale is 4-6 weeks." when I checked in September 2006 it was "... in the pipeline for the coming months".
Entrust - Has a "Entrust Unified Communications Certificates (UCC)" for Exchange Server 2007 and Live Communications Server that can be ordered directly from their website (Source).
Geotrust - Has a "Power Server ID" supporting up to four server names (That is three in SAN's, which in LCS means three in total). Find it here.
If you know other than the above please let me know and I will update the post.