Part of the health care reform bill is a new .9% tax for households with AGI of $250k for MFJ/$200k for single. This tax will be withheld and remitted by employers in situations where an individual's wages exceed $250k in the year. In many cases, it will be the combination of income from a variety of sources which will push the income up to this level.
The .9% tax due but not withheld by an employer will be calculated on/added to tax due on the tax return. Some taxpayers will need to ask to have extra federal tax withheld or make quarterly estimated tax payments to avoid penalties and interest related to not paying in enough tax throughout the year. This new requirement will really hit home in 2014 after people have paid the .9% with their 2013 return. Each year taxpayers at this level must remit 110% of the total previous year tax to avoid penalties.
Taxes which are withheld but not due - perhaps due to business losses of a spouse - will be refunded with the return.