According to the VersionTracker What's New description:

Adds support for Mac OS X 10.4 (Tiger) and additional handhelds that were not supported by version 4.2.1 Rev A. We have devised a more elegant fix for the permissions issue we had encountered with iSync (formerly addressed in Rev C). We have re-released the desktop with the improved installer behavior. For those who have already installed Rev C, there is no reason to update to Rev D. The Rev D installer does not adjust permissions on non-Palm-related folders. However, if you installed Palm Desktop and you noticed that some Mac applications no longer work, you may need to reinstall these applications. See Knowledge Library article 40283 for details. Palm Desktop is a Date Book, To Do List, Address Book, and more. It can be used as a stand-alone application, or to view, sort, find, edit, back up, and add applications to your Palm device. HotSync Manager is the software that controls synchronization between your PC and your Palm device.

Palm Link is here: