As Mark already pointed out, Directory Server 5.2 patch 6 is now available either as a full partial download from the Directory Server Enterprise Edition Download page, or in the form of patches from Sun Solve.
Release notes :
Patch6 Release Notes
Update on October 1st 2007: I stand corrected, DS5.2patch6 is only available as a patch. On the download page, there is also a link to the most recent FULL package on which to apply the patches.