There are two different things happening in 2013 that could affect your paycheck: one relates to social security taxes and the other to income taxes.
Without action, the Bush tax cuts expire at midnight and we revert to the Clinton -era levels. I am 99% sure that a deal will happen thisnweek, that will be applied retroactively, that will preserve the cuts for anyone making around $400k or less, or families making less than $450k. Both sides know that not doing this will be political suicide. As a matter or fact, that part was already agreed- it's the other elements of the cliff that are holding bit up. Once that happens, there will be no change to federal income tax liability for those under the threshold.
The second change relates to social security tax and it WILL affect everyone. It has 2 effects: paychecks in 2013 will decrease by 2% of your gross pay (less pre, but not post- tax dedecutions). Also, the amount of money that you pay SS taxes on will go up. For 2012, once you made $110'100, you stopped paying SS tax. For 2013' the amount goes up to $113k. This portion of the change will not decrease each check, but you will pay it longer into the year.
The SS related changes WILL happen IMHO.