For what it's worth it looks like some (maybe many) of the 5.3 update 
srpms are starting to appear e.g. under

   ftp://ftp.redhat.com/pub/redhat/linux/enterprise/5Server/en/os/SRPMS/

I see the new kernel kernel-2.6.18-128.el5.src.rpm etc.  The announcements 
of the new bits seem to have started...

   https://rhn.redhat.com/errata/RHSA-2009-0225.html
     Important: Red Hat Enterprise Linux 5.3 kernel security and bug fix
     update
   https://rhn.redhat.com/errata/RHEA-2009-0133.html
     A new redhat-release package is now available for Red Hat Enterprise
     Linux 5.3.

etc etc.

I see that they decided to include nedit :-)