Back in October, Microsoft announced that the Release to Manufacturing (RTM) build of Exchange Server 2013 was signed off.
And in 3 Dec 2012 Microsoft announced the Exchange Server 2013 General Availability. So, for those of you who wish to roll out Exchange Server 2013 in Greenfield environments, or in labs and test environments to evaluate new features and test with LOB applications, you can start right away.
But if you are running Exchange 2010 or 2007 environment, you'll need Exchange Server 2010 SP3 or 2007 roll up in order to upgrade. Microsoft is hard at work putting the finishing touches on Exchange 2010 SP3 and the roll up, which are on track for release in Q1 of calendar year 2013.
It's a good idea to evaluate prerequisites and system requirements at this time, as well as become familiar with Exchange's simplified building block architecture, so that you can get a head start on hardware sizing, planning and purchasing.