Class SubversionReleaseSCM$Initializer sets the svnkit.symlinks property to false, if not already set. According to the comments, this was needed as a work around for svnkit issue http://www.nabble.com/Slow-SVN-Checkout-tf4486786.html, which currently is a dead link.
The SubversionSCM plugin had a similar work around, but it was removed July 2009 (in their version 1.3), so I assume the work-around is no longer needed.
The work-around is currently biting me. I have a multi-node build system with one (Linux) master and several (Linux) slaves. In our project, we use symbolic links extensively, and have svnkit.symlinks unset, relying on correct default behaviour.
svn-release-mgr explicitly sets the property (to a value not correct on Linux systems), which is also affecting our SubversionSCM projects, because they also use svnkit, and hence also look at the (now incorrectly set) svnkit.symlinks property.
Work around seems to be to change the command-line for all our slaves to set svnkit.symlinks to true, which is doable, but clumsy. Much nicer would be to remove the (obsolete) workaround from svn-release-mgr.