Not for now...but this have been implemented successfully by the Exchange Team:
As most of you know, Exchange servers utilize the Extensible Storage Engine (ESE) database. As you have told us loud and clear, Exchange rocks from a performance, scalability, and availability standpoint on ESE and we've worked hard to make it so.
However, for some time (it has been a topic of discussion since the development days of Exchange 2003) there have been questions and rumors around replacing the ESE database with SQL Server.
Did the team consider using SQL Server for the next version of Exchange? - Absolutely!
Did it work and perform well? - Yes! Some very smart engineers did amazing work, and we had mailboxes up and running using a SQL Server database.
Did we ultimately decide to replace the ESE database with SQL Server for the next version of Exchange Server - No.
Why not! you ask - It was ultimately determined that the best way to ensure we could drive compelling innovation into Exchange for 2010 and beyond was to remain committed to ESE. Enhancements made to the ESE database for Exchange 2010 have yielded significant performance benefits, High Availability benefits and provided customers with further opportunities to lower their storage costs. As we march forward on a path of developing Exchange for both on-premises deployments and cloud services, there are a number of areas that you are looking to us to make further improvements and new feature investments in the product. After much debate about both
See the full article : http://blogs.technet.com/b/exchange/archive/2009/07/16/3407812.aspx