I noticed a post on Rob Mensching's blog this weekend that I wanted to link to here as well. In the post, located at http://robmensching.com/blog/archive/2007/09/03/Windows-Installer-4.5-and-the-WiX-toolset.aspx, he outlines some work he did this past week to start enabling support for Windows Installer 4.5 in WiX now that the beta of Windows Installer 4.5 has been released.
To summarize that post, Rob checked in the first part of the work necessary to enable Windows Installer 4.5 support in WiX v3.0, but he is going to wait until the final release of Windows Installer 4.5 to enable the equivalent support in WiX v2.0.
Part of the reason for his decision to wait to support Windows Installer 4.5 in WiX v2.0 is an interesting issue with the new msidbCustomActionTypePatchUninstall bit flag being introduced in Windows Installer 4.5 for the Type value in the CustomAction table. Rob outlines the issue in his post, and this issue is also described in more detail in this post on Heath Stewart's blog. If you are evaluating the Windows Installer 4.5 beta, I encourage you to read Rob and Heath's posts before deciding to use the msidbCustomActionTypePatchUninstall attribute and understand the patching implications for previously shipped packages.