Exchange running on SQL back-end is a neat lab engineering idea, one I have thought of in the past, but like the majority of the posts, it would be a SysAdmin nightmare in an operational environment to configure, tune, manage, support, and recover. The storage hardware alone would have to be optimally configured for ultra-quick, simultaneous database reads and writes, and SQL Server continually tuned by an experienced DBA to manage a huge number of indices. In SQL best-practices, the recommendation would be to have an OLTP database for current emails (<7 days?) and an OLAP database for older emails, further compounding the complexity and administrative overhead. Definitely stick w/ ESE!