Microsoft have announced that Exchange 12 is to be 64 bit only:
To help customers take full advantage of the power of 64-bit computing, products including Microsoft® Exchange Server “12,” Windows Compute Cluster Server 2003, Windows Server™ “Longhorn” Small Business Server, and Microsoft’s infrastructure solution for midsize businesses, code-named “Centro,” will be exclusively 64-bit and optimized for x64 hardware.
You can read the full press release here.
It’s not something that I’m going to worry too much about at the moment but I will be making sure that any new hardware we buy will be compatible with the next version of Exchange.
Ben Winzenz gives some detail on what hardware will be compatible in his blog posting.