If you're reading this, chances are you're contemplating Exchange 2003 SP1 mixed-mode Site Consolidation. One of the prereqs for moving public folders, mailboxes, distribution lists, or contacts cross-site with this new functionality is that you have installed a DS/IS hotfix on all of your 5.5 public folder servers throughout the organization. This is a hard requirement and you can't proceed with the moves until all the 5.5 servers have this hotfix installed. The deployment guide and deployment tools helpfile both point you to KB 836489 to get this hotfix.
If, for whatever reason, you can't download the KB 836489 hotfix directly from this KB article, the Exchange 5.5 “May 2004” rollup hotfix available for download at KB 841765 is a suitable substitute (this rollup hotfix actually replaces the KB 836489 hotfix). The KB 841765 rollup hotfix is available for download directly, so no need to call PSS to get it.