However, with no conversion limitations, people with any income can simply contribute to a Traditional IRA and then convert that to a Roth IRA immediately afterwards.
This is great news for those higher income earners who have been previously unable to contribute to a Roth IRA. In addition, if you have been contributing to a non-deductible Traditional IRA in previous years, you can now finally convert those already-taxed funds into a Roth IRA, which is almost as good as retroactively contributing to a Roth IRA. You’ll only have to pay taxes on any gains you earned on the contributions, not the actual contributions themselves since they were already taxed before. (With the recent market performance, that isn’t much of a problem for most of us…)
One additional wrinkle is that if you have a mix of pre-tax and post-tax contributions inside all of your combined Traditional IRA funds, you cannot convert them separately. For example, if you have a mix of 50% pre-tax and 50% already-taxed funds, then any converted amount will be assumed to be 50% pre-tax and 50% post-tax. You can’t just convert the post-tax part. This could be one reason not to roll over all pre-tax 401k funds into a Traditional IRA whenever possible.
More info: IRS Publication 590, “What’s New for 2010″