The formal announcement on NASA's participation or non-participation in the 2016 and 2018 ExoMars missions will come with the release of the President's FY13 budget proposal on February 13. However, BBC News reports that NASA officials have informally told their ESA counterparts that NASA will not be able to participate in either the 2016 orbiter or the 2018 rover mission. The BBC article says that ESA hopes to substitute Russian participation for NASA's to enable the mission. However, without NASA's participation, ESA will have to develop its own descent and landing system which would likely raise the cost to ESA above the current 1 billion euro budget. (ESA's member states have so far committed only 850 million euro).
A separate post on the NASA Watch site says that early leaks on the FY13 budget report that NASA's budget for Mars exploration will be cut in half, although it doesn't state whether that would be immediate or phased in.
I will provide an analysis of the FY13 budget proposal when it is released next week.