Earnings limits for retirement benefits

If you were born January 2, 1943, through January 1, 1955, then your full retirement age for retirement insurance benefits is 66. If you work and are full retirement age or older, you may keep all of your benefits, no matter how much you earn. If you are younger than full retirement age, there is a limit to how much you can earn and still receive full Social Security benefits. If you are younger than full retirement age during all of 2009, Social Security will deduct $1 from your benefits for each $2 you earned above $14,160.

If you reach full retirement age during 2009, Social Security will deduct $1 from your benefits for each $3 you earn above $37,680 until the month you reach full retirement age.

See here for examples :
http://www.ssa.gov/pubs/10069.html#howmuch