When yum updating from f17 to f18 I get Updating : polkit-0.107-2.fc18.x86_64 166/436 warning: user polkitd does not exist - using root warning: user polkitd does not exist - using root - and the polkitd user do not exist after upgrading. Removing the package (with force) and installing it again works correctly and the %pre will correctly create the user. I have no idea why it didn't work while updating.
*** This bug has been marked as a duplicate of bug 841451 ***