Originally, we stated we would deliver Exchange 2013 RTM Cumulative Update 1 (CU1) by the end of this quarter.  Unfortunately, we are not going to meet that goal. We know that many of you will be disappointed as a result of this statement. We understand your pain, however, the decision to delay is due to an issue we found in our final test pass coupled with feedback from members within our Technology Adoption Program community.

Specifically, we found an issue with Exchange 2010 coexistence. The issue actually had an easy workaround, but we made a decision; instead of burdening you with a configuration change on all of your Exchange 2010 Client Access servers, we decided to take a code change in Exchange 2013 and solve the problem so that you will not have to make any additional configuration changes. Given that the goal of CU1 is to enable coexistence with legacy versions of Exchange, we felt this was the right decision; after all, we want to ensure that your upgrade to Exchange 2013 and your coexistence period goes as smooth as possible.

--> Please see the rest on the blog : http://blogs.technet.com/b/exchange/archive/2013/03/25/exchange-2013-rtm-cu1-status.aspx this means that the migration from any Exchange to 2013 will have to wait till April....