For those with a long memory, you may recall the Server Base OS MP 05.0.3500.0000 was withdrawn last October a short time after its release, due to technical issues. The most prevelent issue was that MOM agents could fail to restart on boot. See my earlier ramblings on the subject (http://blogs.technet.com/smsandmom/archive/2006/10/06/MOM-2005-Base-OS-Management-Pack-version-05.0.3500.0000-has-been-withdrawn-due-to-technical-issues.aspx).
The MP is now undergoing re-release and should be available here http://www.microsoft.com/downloads/details.aspx?familyid=E553062F-BD85-4772-8037-8B91F457B710&displaylang=en now, or very shortly (allowing for global replication).
Notes: The 3500 version of the MP is currently only available in English. It is expected that localized versions per the 3100 release for French, German and Japanese will follow (sorry no timeframe right now).
The rules I previously documented as causing the issue have unfortunately had to be removed (http://blogs.technet.com/smsandmom/archive/2006/10/18/workaround-for-those-with-mom-service-startup-issues-after-installation-of-windows-server-base-os-mp-05-0-3500-0000.aspx)
Another change to the MP has been the addition of an R2 servers discovery script which leverages WMI Win32_OperatingSystem to determine R2 servers and add them to a new discovered group. Many of the R2 computer groups then utilize the discovered group and other attributes for specific R2 component computer group membership. This allows determination of 64-bit R2 components by the 32-bit MOM 2005 agent which would have beena problem in the previous version of the MP.