Those aren't the only roadblocks ETFs face. Large corporate plans have access to the lowest-cost mutual funds, negating a large part of ETFs' price advantage. They also have access to collective trusts--very-low-cost institutional accounts managed by banks.

At smaller companies, the obstacle is operating costs. Traditional mutual funds offer an easy way to pass costs along to participants: Many funds have special classes of shares for retirement plans that have extra fees built in to compensate advisors and service providers, as well as paying for record keeping and administration.

Such fees, including 12b-1 fees earmarked for marketing and distribution, can run to 0.5% or more of assets per year. ETFs generally don't have 12b-1 fees, so they don't offer that kind of bundling.

What's more, it's much tougher for third-party administrators of retirement plans to tackle trading and acquisition of ETF shares on behalf of the plan participants, as well as managing the record keeping for fractional-share ownership.

Still, a number of companies see opportunities to offer ETF-based 401(k) plans.

Darek Wojnar, managing director of product strategy for BlackRock Inc.'s iShares unit, the biggest issuer of ETFs, is working with third-party administrators to overcome the largest technical barriers to holding ETFs in the plans. So far, 60 administrators--in an industry of about 1,600 overall, according to Judy Diamond Associates--are set up to handle ETF transactions.

As for investor assets, iShares had $2.7 billion of its overall $352 billion in ETFs housed in 401(k) plans at the end of June.

In March, Folio Investments launched Folio(k), a series of 401(k) offerings structured around ETFs. In the program--as with similar programs from other providers--participants can mix and match ETFs as they wish, or choose among portfolios of index ETFs in specific risk- or age-based allocations.

Several other firms, including ING Direct and Ascensus, are catering to plans looking to incorporate ETFs.

Some smaller players are getting in on the game, too. Brenda Wille-Cope, a managing partner at First Financial Strategies in Denver, moved her company's own plan to risk-based ETF model portfolios constructed by 3D Asset Management of East Hartford, Conn.