The lead article in the August 2006 Journal of Political Economy offers the following abstract:
We solve each household’s optimal saving decisions using a life cycle model that incorporates uncertain lifetimes, uninsurable earning and medical expenses, progressive taxation, government transfers, and pension and social security benefits. With optimal decision rules, we compare, household by household, wealth predictions from the life cycle model using a nationally representative sample. We find, making use of household-specific earnings histories, that the model accounts for more than 80 percent of the 1992 cross-sectional variation in wealth. Fewer than 20 percent of households have less wealth than their optimal targets, and the wealth deficit of those who are undersaving is generally small.
In other words, most Americans are saving enough for their retirements. The authors (John Karl Scholz, Ananth Seshardi, and Surachai Khiatrakun) stress that their data cover only the early 90s, although if anything they believe this biases their estimates downwards by missing out on later capital gains. Here is the paper.
Notes: This result does not deny that America may face coming demographic problems for funding social programs, most of all Medicare. But next time you read that "the U.S. savings rate is zero," think back on this blog post and on that paper.