On May 22, 2007 hot fixes for MSI 3.1 were released under Microsoft Security Advisory 927891 (http://msdn2.microsoft.com/en-us/library/aa371185.aspx). Also see KB article 927891 (http://support.microsoft.com/kb/927891). The hot fixes update the MSI 3.1 files as indicated below. 3.1.4000.4033

Windows 2000
Msi.dll 3.1.4000.4033

Windows XP, 32-bit versions
Msi31.dll 3.1.4000.4039
Msi31.dll 3.1.4000.4039

Windows XP, 64-bit versions
Msi.dll 3.1.4000.4042 (x64)
Wmsi.dll 3.1.4000.4042 (x86)
Msi.dll 3.1.4000.4042 (x64)
Wmsi.dll 3.1.4000.4042 (x86)

Windows Server 2003, x86-based versions
Msi.dll 3.1.4000.4042
Msi.dll 3.1.4000.4042

Windows Server 2003, x64-based versions
Msi.dll 3.1.4000.4042 (x64)
Wmsi.dll 3.1.4000.4042 (x86)
Msi.dll 3.1.4000.4042 (x64)
Wmsi.dll 3.1.4000.4042 (x86)

Windows Server 2003, Itanium-based versions
Msi.dll 3.1.4000.4042 (IA-64)
Wmsi.dll 3.1.4000.4042 (x86)
Msi.dll 3.1.4000.4042 (IA-64)
Wmsi.dll 3.1.4000.4042 (x86)

On 5/27/2008, after applying Service Pack 3 and subsequent updates to a fresh install of Windows XP Professional Service Pack 2, the following files and information were found in %windir%\system32 with the respective file versions and dates modified.

msi.dll 3.1.4001.5512 4/14/2008 5:42 AM
msiexec.exe 3.1.4001.5512 4/14/2008 5:42 AM
msihnd.dll 3.1.4001.5512 4/14/2008 5:42 AM
msimsg.dll 3.1.4001.5512 4/13/2008 9:09 PM
msisip.dll 3.1.4001.5512 4/14/2008 5:42 AM

Will there be a seperate redistributable of MSI 5.0 that can be installed on XP or Vista?

[tfl - 22 04 09] You should post questions like this to the MSDN Forums at http://forums.microsoft.com/msdn or 
the MSDN Newsgroups at http://www.microsoft.com/communities/newsgroups/en-us/. You are much more likely get a
quicker response using the forums than through the Community Content. For specific help about:
Visual Studio  : http://groups.google.com/groups/dir?sel=usenet%3Dmicrosoft.public.vstudio%2C&
.NET Framework : http://groups.google.com/groups/dir?sel=usenet%3Dmicrosoft.public.dotnet.framework
All Public     : http://groups.google.com/groups/dir?sel=usenet%3Dmicrosoft.public%2C&

Per TechNet article http://technet.microsoft.com/en-us/library/dd335033.aspx, Windows Installer 4.5 will be in Vista SP2 and Windows 2008 SP1.