Monday (8-12) Microsoft added SQL Clustering as a supported High Availability solution for Lync Server 2013. Lync Server 2013 was previously support with a SQL Cluster but only if upgrading from a Lync 2010 environment that used a SQL Cluster; in a fresh install of Lync 2013 it was not supported.
Lync Server 2013 can now use either configuration, SQL Mirroring or Clustering, in a Microsoft supported configuration. The advantage and preference is still for SQL mirroring because of the simplified configuration and costs. But if there is already an acceptable SQL cluster, it is probably easier to setup a clustered database vs building out a new SQL Mirror set + witness for HA.
For more information please visit http://technet.microsoft.com/en-us/library/gg398990.aspx