Patches are available from http://www.kfki.hu/~kadlec/sw/postfix_patch.html which make it possible to implement UCE filters in Postfix on a per-user, rather than a global, basis. These are essential for sites where some users need / want to filter mail and others need / want completely unfiltered mail The patches are well-maintained (although the current version posted is against 1.1.4, it still applies cleanly to 1.1.7; new versions are posted whenever revision is necessary) and perform quite well.
Since these didn't make RHL 7.3, it be nice if they were included in the next release....
Closing this -- the advent of spamassassin (and inclusion of that in RHL) is an acceptable alternative....