Forced Upgrade to 64-bit

From a Microsoft Pres Release:

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.

So, if you want these programs I you'll have to buy a 64-bit machine. Start saving your pennies now!