K2 4.6 supports Sql Server 2012
K2 just updated the compatibility matrix to highlight the official support for Sql Server 2012.
In order to use SQL 2012 (for blackpearl and blackpoint) you must be on K2 4.6.
Clean 4.6 installs and upgrades to 4.6 are both supported on SQL 2012.
Always-on & Mirroring features of SQL 2012 are not supported yet.