If you’ve got a TechNet or MSDN subscription, you can get the RTM bits of Exchange Server 2013, Lync Server 2013, Office 2013 Professional Plus and Sharepoint 2013 NOW!
With the RTM bits available, you can start testing the product itself as well as compatibility with 3rd party products or in-house developed solutions. Be advised that a schema update is required to support Exchange Server 2013.
Finally, the updates for Exchange Server 2007 and Exchange Server 2010 required for co-existence are still not available. Exchange 2010 SP3 is expected first half of 2013; no details have been disclosed on the update for Exchange Server 2007.