As mentioned before - we now have CTP build of Exchange 12 those of you that have MSDN and TechNet subscriptions can install it.
One issue that seems to come up frequently for those of you that have tried installing this build is the following error at Setup:
"Exchange Server '12' requires Microsoft Management Console 3.0."
This happens even though the final (RTM), released version of MMC is installed on the system.
The problem here is that the CTP build of Exchange 12 setup actually needs a "RC1 refresh" build of MMC 3.0, and as this build is not found - the setup fails with the above error. E12 is looking for a registry key that is not there on the machine that has RTM version of MMC 3.0 installed. This issue is going to be fixed in later builds of Exchange 12 and will definitely not be there in final release version.