The Exchange development team has given us the authoritative word on using current versions of Exchange on 64-bit versions of Windows. In short, that word is "No." However, the way the worded their response left one slight ambiguity, so I used the comments to ask -- and receive -- the answer.
Summarized:
- You cannot use Exchange on a 64-bit version of Windows Server 2003 (x64 or IA64).
- You can use Exchange on a 324-bit version of Windows Server 2003 that is running on 64-bit hardware (necessarily x64, since the Itanium isn't backwards-compatible).
What holds you back? The Installable File System (better known as the infamous M: drive), which requires a kernel module. This module is a 32-bit driver.
Using the 32-bit version of Windows on a 64-bit machine of course places limits on how much of a performance increase you'll get out of your spiff new Opteron server (only 4GB RAM, etc.), but what it does do is get you nicely placed to upgrade to E12 when it comes out. According to Paul Thurrott's sneak-peak at E12, 64-bit support is definitely in the offing:
Additionally, E12 will ship in various x64 versions, which are compatible with new 64-bit hardware platforms, including machines based on the AMD Opteron processors and Intel's Xeon EM64T designs. Although Microsoft hasn't committed to specific Exchange editions as I write this, I've learned that each Exchange edition would ship in both 32-bit x86 and 64-bit x64 versions.
Good stuff, as there have been persistent rumors that E12 would only offer 64-bit support. As much as I think 64-bit hardware (now that it has finally hit the Windows world; the UNIX world has had 64-bit hardware and operating systems for years!) is the only sane choice for the 3-5 year roadmap, you have to have OS and application support for it and vendors should not be dropping support for 32-bit platforms until 64-bit support is more ubiquitous and stable.