From the mailbag today:
Question: It is my understanding that you can’t run 32-bit Exchange on 64-bit Windows Server 2003. Is that correct?
On 64-bit hardware w/32-bit OS, yes.
On 64-bit hardware with Windows Server 64-bit OS, no.
"The x64-based versions of Windows Server 2003 seamlessly support the running of 32-bit programs. However, on the x64-based versions of Windows, all the kernel mode components must be native 64-bit components. Therefore, all drivers must be 64-bit components.
User mode 32-bit programs are run under Windows on WOW64. WOW64 is the 32-bit emulator. However, Exchange Server 2003 will not even be installed correctly under WOW64. The primary reason is that Exchange Setup installs the 32-bit driver for the Installable File System (IFS) that is incompatible with the 64-bit kernel.
Note The information described here does not apply when you run Exchange Server 2003 on 32-bit versions of Windows Server 2003 that are installed on 64-bit hardware. You can do this on 64-bit hardware that also natively supports the running of 32-bit operating systems and programs."
Source: Microsoft KB 924046