Microsoft today released Service Pack 3 for Exchange 2010 (available from http://www.microsoft.com/en-us/download/details.aspx?id=36768).
SP3 enables full coexistence between Exchange 2010 and Exchange 2013, plus it adds a couple of new features, such as Windows Server 2012 support. See the full feature list here.
After you’ve upgraded allof your 2010 servers to SP3, you will be able to install Exchange 2013 in your forest. Opinions differ on whether you should– for example, fellow MVPs Tony Redmond, Michael B. Smith, and Jason Sherry all agree that you should wait. For my part, I’m OK with deploying 2013 into production; it does have some rough edges, but I think CU1 is right around the corner, so adding 2013 in your environment now is certainly defensible.