The Paris Observatory has announced a leap second between June 30th and July 1st this year:
A positive leap second will be introduced at the end of June 2012. The sequence of dates of the UTC second markers will be:
2012 June 30, 23h 59m 59s
2012 June 30, 23h 59m 60s
2012 July 1, 0h 0m 0s
... Leap seconds can be introduced in UTC at the end of the months of December
or June, depending on the [available rotation data].
Leap seconds occur from time to time because the earth's rotation on its axis doesn't stay exactly the same from year to year. Most years it loses about half a second; the last couple of years it hasn't lost as much, so the last leap second came just before 1 January 2009. Eventually, the earth will stop rotating on its axis relative to the sun, in much the same way the moon rotates once on its axis every time it orbits the earth. You've been warned.
This has an interesting side effect, by the way: UTC is now 34 seconds behind the earth, so clocks on things like orbiting satellites—think GPS—have "incorrect" values. Your hand-held GPS receiver will probably be a second slow after June 30th. Your computer, if it syncs up to an authoritative time service, won't.