After several years of preparatory work during the Bush administration, the Obama administration's EPA is expected to announce by June 26, 2009, its proposed primary standard for one of the six "criteria" air pollutants, nitrogen dioxide. The standard for NO2 is intended to address other compounds of nitrogen and oxygen that typically mingle with NO2, which serves as an indicator for the mix of nitrogen oxides.
EPA released its underlying science for NO2 on July 14, 2008. As usual, it is up to the EPA administrator to select a standard, using the science as one starting point. Whether the selected standard is based on the science and/or politics and economics, and whether it is protective enough, are often points of heated debate among various interest groups.
For extensive information on the NO2 standard and the underlying science and politics, see the TipSheet of July 23, 2008. 
Health advocates such as Clean Air Watch's Frank O'Donnell,  202-302-2065, and the American Lung Association speculate that the current EPA annual standard of 53 ppb (which is 2.5 times higher than the World Health Organization'sguideline) may not be changed much, but would like to see the anticipated new 1-hour standard be about 50 ppb, and no higher than 100 ppb, either of which likely would be more protective than the current annual EPA standard. EPA's Clean Air Scientific Advisory Committee has recommended that the 1-hour standard not exceed 100 ppb,  which would be very similar to the World Health Organization's 1-hour guideline. The 2008 EPA science report made recommendations for the 1-hour standard ranging from 50-200 ppb.
The proposed rule for the new primary standard should be available here. 
The final primary standard is scheduled to be released by Jan. 22, 2010, per court order (which extends the original court-ordered date of Dec. 18, 2009). The June 26, 2009, release is also a court-ordered date (extended from May 28, 2009).
Release of the proposed and final rule for the secondary NO2 standard, which targets protection of the environment rather than health protection that is the focus of the primary standard, is trailing each of these dates by several months. It is scheduled to be released in conjunction with a secondary standard for sulfur oxides, with a proposed rule on Feb. 12, 2010, and a final rule on Oct. 19, 2010.
For more on the sulfur oxides primary and secondary standards, see the TipSheet of Sept. 3, 2008.