Visit Efficient Windows Organization for more information on this and many more articles.

As a result of the stimulus bill, homeowners can claim a tax credit of up to $1,500 for upgrading their primary residence with energy efficient appliances and building components, such as exterior windows, doors and skylights. This way, homeowners can reduce their income tax by up to thirty percent of the purchase price
(not including installation costs) to qualified products installed in 2009 and 2010.
The tax credit helps you save money on the purchase of windows, doors or skylights, but the real money savings are what you’ll save in the long term with reduced energy bills. Upgrading to energy-efficient windows and doors can save home-owners hundreds of dollars in heating and cooling costs each year.* And, of course, more efficient windows and doors also substantially enhance comfort, reduce condensation, and help to protect the environment and conserve limited resources.

What are the qualifying criteria?
U-factor <= 0.30, SHGC <= 0.30
In order to qualify, windows, doors, and skylights must be accompanied with a signed statement from the manufacturer that certifies that the product complies
with the tax credit criteria. Qualified windows, doors, and skylights need to have NFRC-certified U-factor and Solar Heat Gain Coefficient (SHGC) ratings that
do not exceed 0.30.

These criteria apply throughout the United States. A low U-factor indicates
good insulating value, which is particularly important in cold climates,
whereas a low SHGC indicates less solar heat gain, which is particularly
important in climates with high cooling demand. U-factor and SHGC ratings
can be found on the NFRC label. An example of the label is shown on the
left (although this example would not qualify for the tax credit).

Homeowners should keep the manufacturers certification statement with
their records but are not required to submit it with their tax return.