RTFA: http://tf.nist.gov/timefreq/pubs/bulletin/leapseco…

The master clock pulses used by the WWV, WWVH, WWVB, GOES, ACTS, and Network Time Service NTS time code transmissions are referenced to the UTC NIST time scale. Occasionally, 1 s is added to the UTC time scale. This second is called a leap second. Its purpose is to keep the UTC time scale within ±0.9 s of the UT1 astronomical time scale, which changes slightly due to variations in the rotation of the Earth.
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The first leap second was inserted into the UTC time scale on June 30, 1972. Leap seconds are used to keep the difference between UT1 and UTC to within ±0.9 s.
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NOTE: A positive leap second will be added at the end of December 2008.

And for a little more info:

RTFA: http://tf.nist.gov/timefreq/general/leaps.htm#Anch…

What is a leap second?

A leap second is a second added to Coordinated Universal Time UTC to make it agree with astronomical time to within 0.9 second. UTC is an atomic time scale, based on the performance of atomic clocks. Astronomical time is based on the rate of rotation of the earth. Since atomic clocks are more stable than the rate at which the earth rotates, leap seconds are needed to keep the two time scales in agreement.

The first leap second was added on June 30, 1972. Since then, they have occurred at an average rate of less than one per year. Although it is possible to have a negative leap second a second removed from UTC , so far, all leap seconds have been positive an extra second has been added to UTC . Based on what we know about the earth s rotation, it seems unlikely that we will ever have a negative leap second.

Why do we need leap seconds?

Leap seconds are needed so that users of the astronomical time scale UT1 can use UTC and know that the difference between the two time scales is never greater than 0.9 seconds. Currently the difference between UT1 and UTC is changing at a rate that requires the addition of less than one leap second per year. When a leap second is necessary, an announcement is made at least several months in advance, and all leap seconds so far have been implemented on either June 30th or December 31st.

Oh HELL yes. 2008 was a great year, and we’re getting a little extra time to savor it. If you like to party hardy, international time geeks are giving you one second more of the fun before ringing in this New Year … you better make it good!