I read yesterday that the leap second was handled by repeating the last second of the day. So you would see 59 seconds for two seconds but never 60 seconds. I guess different systems and time standards may handle it differently.
I'm sure that's how some systems implement it and it's close enough to the truth that I'm sure it will frequently be reported that way in the news. But I'm also quite sure that the actual standards are as I said they were. See the wiki link for a detailed timeline.
