At Vanguard, only two dozen out of 2, 500 plans it administers are offering the conversion option for 2010.

FORBES: Roth Conversions In 401(k)s For Real In 2010

He says a lot of big employers and record keepers felt it was too late to offer the in-plan conversion option this year.

FORBES: Roth Conversions In 401(k)s For Real In 2010

For starters, the majority of defined-contribution plans haven't added the Roth 401(k) in-plan conversion option since it was created by a federal law nearly two years ago.

WSJ: Family Value: Is It Time to Convert

If you qualify for a conversion, and if you avail yourself of the option, you will probably save a bundle for yourself or your heirs.

FORBES: Rothify now

With the new in-plan conversion move, if your plan already has a Roth option, you can do a rollover this year.

FORBES: Turn Your 401(k) Into A Roth To Beat Tax Hikes

Second, a special provision Congress adopted (as a budget gimmick) allows those who convert in 2010 (and 2010 only) a special option: they can either recognize all their income from the conversion on their 2010 tax returns or spread that income over the 2011 and 2012 tax years.

FORBES: Obama Tax Deal Would Sweeten Roth Conversions