I just noticed a set of steps that have been posted on the Windows Installer team blog that can be used to sign up for the Windows Installer 4.5 beta.  If you are interested in trying out this beta and haven't signed up yet, I encourage you to check out http://blogs.msdn.com/windows_installer_team/archive/2007/08/21/walkthrough-for-signing-up-for-windows-installer-4-5-beta.aspx for more information.

Currently, signing up for the 4.5 beta will allow you to access some white papers about agility in software packaging and how Windows Installer 4.5 can help achieve agility goals.  In addition, according to this post, the Windows Installer 4.5 beta will be available for download "any day now" to members of the beta program.