While Microsoft officially calls Windows 8.1 an update to Windows 8, which shares the same support lifecylce (see http://support.micro...cycle/?p1=17655), from Windows Installer's perspective it's is a whole new version, not a service pack, as indicated by the following property values:
VersionNT = 603 (was 602 in Windows 8)
ServicePackLevel = 0
MSI itself now has version number 5.0.9600.16384 (was 5.0.9200.16384 in Windows 8). Apparently, only the build number of Windows 8 (9200) was replaced with that of Windows 8.1 (9600). Therefore I don't expect new functionality.