What does the tax credit cover?
According to the National Association of Home Builders (NAHB), the tax credit for efficiency upgrades in existing homes (Internal Revenue Code Section 25C) is available for 30 percent of the cost, up to a $1,500 limit in 2009 and 2010, for the installation of certain types of insulation, windows, roofs, water heaters, heat pumps, air conditioners and furnaces.
There's also a tax credit with no upper limit for geothermal heat pumps, small residential wind turbines, and solar energy systems. This credit is good until December 31, 2016, and can be used for existing homes and new construction. Both principal residences and second homes qualify, but rental properties do not.
You can read more details on the kinds of products that qualify and instructions for obtaining the tax credits and rebates at nahb.org and energystar.gov.