The Congressional Budget Office has released Utilization of Tax Incentives for Retirement Saving: Update to 2003.  Here is part of the Preface:

[This] paper examine[s] participation rates and contributions to employment-based retirement plans, individual retirement accounts (IRAs), and self-employed plans. …Overall, participation in tax-favored retirement plans remained stable between 2000 and 2003. The slight increase in 401(k) participation was offset by a small dip in IRA participation. The EGTRRA provision that affected the largest number of IRA participants was the increase in the general contribution limit from $2,000 to $3,000, which allowed an additional 12 percent of participants—1% of all workers—to contribute as much as they wanted. The provision that affected the largest number of 401(k) participants was the introduction of $2,000 “catch-up” contributions for those age 50 and over.