As announced at the sold out Microsoft IT Forum 2005 in Barcelona, Spain, Exchange 12 will ship exclusively for 64 bit hardware.
help customers take full advantage of the power of 64-bit computing,
products including Microsoft(R) Exchange Server “12”, Windows Compute
Cluster Server 2003, Windows Server(TM) “Longhorn” Small Business
Server, and Microsoft's infrastructure solution for midsize businesses,
code-named “Centro”, will be exclusively 64-bit and optimised for x64
[MS Exchange Blog]