MSI 4.5 documentation available publicly
As announced in the Windows Installer Team Blog, the documentation for Windows Installer 4.5 (beta) is now avilable on MSDN, without need for registering on Microsoft Connect and accepting any NDAs.
Information about MSI 4.1 has been pulled from MSDN. It's currently unknown if that means that Microsoft cancelled the plan to ship MSI 4.1 in Windows Vista SP1 and Windows Server 2008.