There is a federal tax credit ($150) for installing new high efficiency furnaces between Jan 2006 and Dec 2007. The catch is that the furnace needs to be 95% AFUE. That seems a tad high (Energy Star qualification is 90%) and removes a lot of condensing furnaces from the credit, including all of American-Standard/Trane condensing gas furnaces.
List of eligible furnaces:
There is also a tax credit for "electrically efficient furnaces" This credit is $50 and is for furnaces whose fan uses no more than 2% of the total energy use per year, regardless of AFUE.
A list of eligible electrically efficient furnaces is here: