As reported before on Bink.nu, Exchange 12 will be 64bit only, now it is officially confirmed.
By the time Exchange 12 comes out only 64bit server hardware will be available anyway so it seems obvious. Eileen Brown blogs about the performance increase and I/O decrease: The product team have been testing E12 on 32 bit and 64 bit, and have found some significant gains and a reduction of IOs per second which results in really good performance gains. They tested Exchange on 64 bit and found almost a 75% reduction in IOs per second compared with Exchange 2003. This could result in almost a 4X increase in the number of users on the same disks or require 1/4 the disks to support the same users from a throughput perspective. If you think about this, it's quite significant since it will proportionately decrease the investment in storage which accounts for ~80% of the capital cost of Mailbox servers. And thats worth noting. I also unserstand that during the betas, there will be both a 32 bit and 64 bit version of Exchange 12 so you can compare the performance gains. The other useful thing is that when you're upgrading to 64bit Exchange 12 you won't need to upgrade your global catalog servers to 64 bit as well. It's a good idea to do the upgrade though and take advantage of the massive IO reduction. And as E12 will support mixed 32 bit and 64 bit environments including legacy Exchange 2003 and Exchange 2000 servers. You can also connect to either 32 bit or 64-bit Outlook. More at source