Exchange 2007 (former Exchange 12) CTP require MMC3.0, even if it is installed Exchange installation can halt because it is not found. The cause is that Exchange 2007 CTP is looking for a key in the registry that only MMC 3.0 RC1 refresh will create. The RTM version of MMC 3.0 do not have this key, thats why the installation halts. It’s easy to fix, just create a empty regkey

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\
Windows\CurrentVersion\Uninstall\MMC30Core

I have upload screenshots of the issue and also screenshots of the work around here