I have a brand new install of WPMU on a Mac OS X server (10.3.9) and WPMU seems to be running just fine.
I installed the site wide tag plugin 0.3.1 I get the Global Tags option under Site Admin/Options.
I click the enabled checkbox and hit Update Options - no errors, but the checkbox never stays selected.
The Global Posts blog is created no problem http://careo.prn.bc.ca/wpmu/tags/
But, posting from other blogs do not show up there.