The team over on the Exchange Blog (You Had Me at EHLO…) have just posted a podcast interview of Terry Myerson talking about Microsoft's decision to only offer Exchange 12 on 64 bit.

Direct links to the podcasts (mp3) (wma).

Previously published blog post regarding Exchange 12 and 64 bit: http://blogs.technet.com/exchange/archive/2005/12/29/416613.aspx.

I hope this helps you as you are planning your future hardware purchases and Exchange upgrades.