Yes, there is a tax credit for windows placed in service in 2009 and 2010. From January 1 - May 31, 2009 all ENERGY STAR qualified windows can qualify for the tax credit.
Beginning June 1, 2009, to qualify for the tax credit, windows (and doors and skylights) must have a U-factor and Solar Heat Gain Coefficient (SHGC) less than or equal to 0.30. This is a super effiicient window, and most ENERGY STAR windows will not qualify.
You can find the U-factor and SHGC on the National Fenestration Rating Council (NFRC) label. NFRC is the only federally recognized organization for determining the energy performance of windows, doors and skylights. Please see the NFRC website for information concerning product performance.
This stringent criteria was put in place by the American Recovery and Reinvestment Act, signed into law on February 17, 2009.