I have recently been hearing from various sources that Exchange 12 will only be available for the 64bit Windows Server platform and not the 32bit platform, I cannot quote anyone yet but as more new comes to hand I will publish the relevant links here. 

UPDATE:
Here are some links and blog posts on the future release of Exchange 12 and it exclusively as 64-bit:

Nathan Mercer's blog post:  http://blogs.technet.com/nmercer/archive/2005/11/18/414746.aspx
Microsoft Press Release:    http://www.microsoft.com/presspass/press/2005/nov05/11-15ITForum05UmbrellaPR.mspx
MS Exchange Blog            http://hellomate.typepad.com/