As the first members of the baby-boom generation enter middle age, concern is mounting that millions of Americans in this huge population group won't be able to retire in comfort. Social Security payments will only meet a small part of most retirees' financial needs. The other traditional sources of retirement income -- employer-sponsored pension plans and personal savings -- may be less secure. Pension coverage for workers is about as broad as ever, but the kinds of pensions available are changing drastically, as cost-conscious employers shift the burden of saving for retirement from the company to the worker. Financial advisers are warning today's workers to start saving early for retirement if they want a secure old age.