Customers are asking me for advice for new Exchange installations and migrations. Until Microsoft releases a rollup (Exchange 2007) or new servicepack (Exchange 2010) it’s not possible to do co-existence migration from 2007/2010 to 2013. My advice is to wait or migrate to 2010.
The only solution now is to export/import using a big bang scenario and that’s for the most company’s not a solution. For new installations you can choose for Exchange 2013 although.
It’s not clear when Microsoft plans to release the update or service pack..