It seems we have some badly worded docs on some Microsoft sites that may lead you to believe that you cannot run Exchange 2003 on x64 bit systems. Also the release notes seem to be in error as well. Let me assure you that this information is incorrect and is being corrected.
You can run Exchange 2003 on x64 systems just fine. You do have to run Exchange 2003 on a 32 bit Windows version but they both run without issue on x64 systems.
I just wanted to post this blog entry so you are not confused and think that you are locked in. If you buy an x64 bit server now you can run Exchange 2003 now and be all the more ready for a E12 64 bit version when it arrives. In fact I would recommend if you are going to buy any systems in the near future that you buy x64 systems as they will not only run Exchange 2003 now, but they will also have a slight performance improvement vs. a 32 bit system and you will be ready (hardware wise) to go to the next version of Exchange that supports 64 bit without having to buy a new system.
Fact is your system maybe x64 bit now and you may not have notice! If you have a system that uses an Intel chip you can use this tool found at Intel’s web site that will tell you if your system is x64 bit capable http://support.intel.com/support/processors/tools/piu/sb/CS-014921.htm if your system has an AMD chip go to the AMD site and download their tool its call AMD CPUID http://www.amd.com/us-en/Processors/ComputingSolutions/0,,30_288_809_871^9706,00.html
Or you can just check by CPU type (not always clear however) you need to have one of these procs: AMD Athlon 64, AMD Opteron, Intel Xeon with Intel EM64T support, Intel Pentium 4 with Intel EM64T.
On the related subject - if you are wondering if Exchange 2003 can run on 64-bit version of Windows, please go here.