Until you change jobs or retire, you typically can't roll your 401(k) into a Roth IRA. And you can't convert your plan into a Roth 401(k). But between jobs or at retirement (at 55 or older), you're free to switch your 401(k) to a Roth IRA as long as your income allows it ($100,000 or less until 2010). New in 2008: You no longer have to first roll it over into a traditional IRA and then convert it. You can now go directly.
NEXT: How do taxes work with Roth conversions?