Lync Server 2010 introduced the idea of user registration happening on the front-end servers (in an Enterprise Pool) instead of the pool database back-end. Standard Edition servers also record user registration in a 'front-end' instance of SQL called the "RTCLOCAL" instance. If you look at the Services MMC snap-in for an Enterprise Edition front-end server you'll notice a SQL Server service for the RTCLOCAL instance.
This instance is where user registrations are held, more specifically they are stored in the "rtcdyn" database within this instance.
To obtain client version data we can actually use the Snooper utility which ships with the Lync Server 2010 Resource Kit Tools. This tool can be found under C:\Program Files\Microsoft Lync Server 2010\ResKit\Tracing. Simply run this tool and select "Reports", then "Conferencing and Presence" from the menu.
In the "SQL Backend" text box type the name of one of your front-end servers if you're using Lync Server 2010 Enterprise Edition followed by "\rtclocal". If you're using Standard Edition, type the server name followed by "\rtclocal". For example:
Change the report type to "Diagnostic" and click the "Generate Report" button. Scroll down to the client version summary section to view the results.
To view client version information for other front-ends in the pool, simply change the server name and query them individually.
Doug's Blog on retrieving users and versions: http://blogs.technet.com/b/dodeitte/archive/2011/06/15/how-to-get-a-list-of-client-versions-and-the-users-logged-into-them.aspx