I would assume that all servers using ntpd will be just fine. Besides any Kerberos issues usually do not show up until 5 minutes out of sync.

@donmontalvo, thanks for bringing this up. It seems that Java on Linux may be impacted:

http://stackoverflow.com/questions/11283271/jvm-always-beyond-100-cpu-usage-in-centos-6

The fix in 2012 was to bounce ntp:

ntpd stop; date; ntpd start

Not sure if this will be needed this time around, since there was a kernel patch to address the issue. Perhaps older Linux distros should be updated ahead of 6/30 as a precautionary step? I am concerned about my NetSUSes that are on Ubuntu...

It appears that there is no issue on RHEL, as long as you are using ntp:

https://access.redhat.com/articles/15145

Interesting, so I guess a fix could be a reboot just before 23:59 on June 30, then once they're back up, they should be good.

If I remember correctly the leap second made no Problems on Windows Systems.
But around 2% of our Linux Machines died because of a Kernel Bug (which is said to be fixed).

So I guess you will be fine!