In order to get tax rebate on central A/C units, make sure you purchase an air conditioner that is rated 1st tier (pdf file) by the Consortium of Energy Efficiency. Also any A/C unit that is energy star qualified (look for the logo) will work as well.
This comes from the new economic stimulus package signed into law in February that revived and expanded tax credits for energy efficient home improvements.
Here’s what is available: tax credit for 30% of the cost of energy-efficient air conditioners, furnaces, heat pumps and boilers, up to a life time cap of $1500. This credit is available for 2009 and 2010.
NOTE: The tax credits you earn on all the energy efficient home improvement items such as central air, water heaters, furnaces and hot water boilers, windows, doors, roofing and installation, are cumulative, and capped at $1500.
Be aware: this tax credit applies to primary residence only. Need an expert for your air conditioning? Hire it Done with Adam Helfman at www.hireitdone.com