WASHINGTON D.C. (CNNMoney.com) - More than 350 S&P 500 companies still have pension plans...and thanks to the recent rise in interest rates and stock prices, about one-half of those plans are at least fully funded, according to Ron Gebhardtsbauer, senior pension fellow at the American Academy of Actuaries.

But that doesn't necessarily mean the pace of freezes in defined benefit pension plans will slow.

Gebhardtsbauer was speaking Monday at a pension conference sponsored by the AARP and the Employee Benefit Research Institute.

Aside from funding issues, there are many other factors leading companies away from offering defined benefit pensions.

According to Clyde Beers, an actuary and principal at Towers Perrin, who also spoke at the conference, about 30 companies among the Fortune 500 are facing a situation where there are more than 8 retirees per worker and -- like their competitors -- they are confronting the fact that workers are living longer.

Some have already moved to freeze their defined benefit pension plans, meaning new workers would not be included. Those workers, however, likely would be included in a defined contribution plan, like a 401(k).

Companies that have maintained their pension plans face new questions. Among them, what will pension reform legislation -- currently being debated in Congress -- mean to them.

Under consideration are proposals that would change the way a company would be penalized for being under-funded combined with new accounting rules that change the way funding is calculated.

Add to that the concern that companies awarded government contracts now may be told -- as some were last month by the Department of Energy -- that federal government will no longer pay for new hires' pension benefits.

And companies are getting mixed signals from court rulings on pension cases.

One of the reasons that companies cite for keeping their defined benefit plans is that they are good recruitment and retention tools, Beers said.

But now that a third of large companies have transitioned to defined contribution plans for new hires, that may signal to other companies that they can remain competitive just by offering a 401(k).

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