This article in the Boston Globe gets the facts right.
Although Mitt Romney pays a lower effective tax rate, it's the same capital gains tax rate that everyone would pay from long term investments.
More of his income comes from investments, than from salary.
Claims against Mitt Romney in new Priorities USA ad are technically true but paint an incomplete picture - Political Intelligence - A national political and campaign blog from The Boston Globe - Boston.com:
Claims about Romney’s high income and low tax rate also are true, but many affluent Americans pay lower effective tax rates than their middle-class counterparts. Because capital gains on investments, where Romney and other top earners make much of their money, are taxed at a 15 percent rate, it is possible for wealthy people to achieve effective tax rates that are far lower than their 35 percent income tax bracket.Here's the video linked to in the article: