In 2008 and 2009, you will be permitted to directly roll over/convert a 401(k) into a Roth IRA as long as you’re willing to pay the taxes for the conversion and if your annual income does not exceed $100,000 (in 2010, the $100k rule expires). In 401(k) official terms, this means that you can convert a distribution from an employer-sponsored plan directly into a Roth IRA. For those keeping score at home, this may come as a surprise since you couldn’t do this in the past and because you are basically throwing all the Roth IRA contribution limits out the window. Currently you can only contribute $5,000 a year to your Roth IRA, subject to income phaseout limitations, and so being able to convert from a 401(k) into a Roth is something that would circumvent that since the 401(k) limits are $15,500 a year!
What happens when you convert? Easy, the amount that you convert will be subject to your marginal income tax rate but then it’s tax-free at disbursement, since it is a Roth IRA at that point. Do you have to convert the whole thing? No, you pick how much you want to roll over, just as you would if it were to go into a Traditional/Rollover IRA, and you only pay taxes on that which converts into a Roth IRA. If you are willing to wait until 2010, you can convert as much as you want and spread out the tax liability over 2010 and 2011.
Now, before you jump at the opportunity to convert everything over to a Roth, I would investigate your tax diversification profile because having all your retirement assets in tax-free is just as bad as having all of your assets in tax-deferred accounts. Make sure you are properly diversified from this perspective so you don’t get sticker shock down the road.