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.