Yes, using 401(k) to build your nest egg is a must-have for the middle class. However, I don't think the study is conclusive. How much of the growth is attributable to the continued salary deduction (vs. portfolio appreciation)?

For workers who saved consistently in their 401(k) plans between 1999 and 2005, the average 401(k) balance rose 50 percent to $102,014, according to a study released Thursday.

The study, conducted by the Employee Benefit Research Institute and the Investment Company Institute, attributed the growth in balances to workers' consistent contributions throughout the six-year period, which included one of the worst bear markets in over 70 years, combined with strong equity returns on those contributions since 2002.

The average account balance in 2005 among workers in their 20s who have had accounts since 1999 was $24,169. For those in their 30s, it increased to $50,930, while for those in their 40s it rose to $91,848. Workers in their 50s had an average account balance of $127,766, while the average for those in their 60s was $140,957.

In terms of asset allocation among all 401(k) account holders, 68 percent of 401(k) assets are invested in stocks - either stock funds or the stock portions of balanced funds, which also invest in bonds.