Update 1/30/13 - Lync Server 2010: January 2013 Cumulative Updates have been released which fixes this issue. You can download the latest updates here: http://support.microsoft.com/?kbid=2791665.
I ran into this issue at a customer the other day. They are in the process of migrating from OCS 2007 R2 to Lync Server 2010. They discovered that after moving some pilot users to Lync Server 2010, if a user tried to join a Live Meeting meeting that one of the pilot users had created either before they were moved to Lync Server 2010 or before the Lync 2010 client was installed by using the Conferencing Add-in for Microsoft Office Outlook, they would get the following error:
Live Meeting cannot connect to the version of server. To connect using Communicator instead, click OK. To cancel out of joining the meeting, click Cancel.
Currently this is being caused by an issue in Lync Server 2010 after applying the Cumulative updates for Lync Server 2010: October 2012. If the user clicks the OK button, the Lync Server 2010 join launcher will launch and use the Lync Web App or the Lync Attendee client to connect to the meeting. If the user already has the Lync 2010 client, after they click the OK button, the Lync 2010 client will open and connect to the meeting. Once the user joins with a Lync client (Lync Web App, Lync Attendee, or Lync 2010), they can participate in the meeting. The clients will have a reduced set of features available during the meeting. Depending on the client used, IM, Audio, Video, Desktop/Application Sharing, and recording are available.