The Exchange Team has a great post clarifying that Exchange 2003 IS supported on x64 hardware, just not on a 64 bit OS.
...There was some confusion as to whether Exchange 2003 would run on x64 hardware. Customers were understandably frustrated because they were recently informed that Exchange 12 will only run on 64 bit hardware. Fret not, buy 64 bit hardware now and use a 32 bit OS until E12 time. I have been running EX03 on my 64 bit laptop for months without problem. IMO, you shouldn't buy a new machine that isn't x64 anyway...
...I publish my first link with the added space in honor of a friend who has an interview coming up. Good luck, MW!
UPDATE 1/4/05 - Today is MW's first day here at MS! Congrats!