A leap second will be introduced at midnight on June 30, 2012. The last time this happened was on the New Years Eve in 2008.
One might say, “OK, I will get to the next appointment 1 second ahead, good!”. So who cares? System administrators would take the matter seriously though. In fact, you may remember a story of Linux machines crashing at the last occurrence of a leap second [1]. It was due to bad code in the kernel prior to 2.6.29 and many OSs were affected including RHEL 4 and 5 [2].
The code has been fixed since. Make sure your kernel is newer than kernel-2.6.18-164.el5 (RHEL/CentOS/SL). RHEL/CentOS/SL 6 kernels are not affected.
A quick note to add is that the above issue applies only if you are running NTP. If the system is not running NTP, you need to correct the clock manually.