Mitel released a doc yesterday in response to questions they were getting regarding the leap second update due June 30th.
It's doc 15-1263-00139
I'm still checking it out but it would appear that if your MSL servers is using NTP there may be a problem
Apparently there are 3 things that may happen
1) Systems hang due to leap-second live-lock
2) RHEL 6 - After the insertion of the leap second several processes, notably java, were reporting a high CPU usage
3) Using -x NTP option still results in instantaneous clock changes when leap second occurs
Ralph