The employer will be obliged to operate payroll deductions on it in the period when the payment is made - so they will tax the pay in January 2013.
However, the individual taxpayer is taxable on income in the period when it is earned i.e. 2012. It will be up to the taxpayer to sort that out with Revenue if they think they'll benefit from it. Otherwise just let it lie in 2013.
In practise this doesn't happen and would be near impossible to implement.
People receiving paycheck on say 4th Jan 2013 are being paid for the week ending 30th Dec. The payroll system will still be in 2012 mode when that week is being processed on say 2nd Jan.