According to a Pew Research Center survey, more than three quarters of today's workers (77%) expect to work for pay even after they retire and, of those, most say it's because they'll want to, not because they'll have to. However, the Pew report--Working After Retirement: The Gap Between Expectations and Reality--suggests that these expectations are dramatically out of step with the experiences of people who are already retired.
Pew reports that just 12% of retirees are currently working for pay (either part or full time), and another research organization shows that just 27% of them have ever worked for pay while in retirement. Pew also shows a disparity between the age at which today's workers say they plan to retire and the age at which today's retirees actually did retire, with the average worker expecting to retire at age 61, while the average retiree actually retired at 57.8.
Source: Pew Research Center Social Trends Report Summary (September 20, 2006)