Excerpt: EXCHANGE-TRADED FUNDS are the current rock stars of the investment world. Assets held in ETFs topped $330 billion by March 31, according to Citigroup, up from $16 billion in 1998. Considering the wider acceptance they're gaining among individual investors, it's not surprising that some firms are trying to push ETFs into the lucrative arena of 401(k) retirement plans, long the domain of mutual funds.