From the US Customs & Border Protection website:
Alcoholic Beverages: One liter (33.8 fl. oz.) of alcoholic beverages may be included in your exemption if:
You are 21 years old.
It is for your own use or as a gift.
It does not violate the laws of the state in which you arrive.
Federal regulations allow you to bring back more than one liter of alcoholic beverage for personal use, but, as with extra tobacco, you will have to pay duty and Internal Revenue Service tax.
While Federal regulations do not specify a limit on the amount of alcohol you may bring back for personal use, unusual quantities are liable to raise suspicions that you are importing the alcohol for other purposes, such as for resale. CBP officers are authorized by the Bureau of Alcohol, Tobacco and Firearms (ATF) to make on-the-spot determinations that an importation is for commercial purposes, and may require you to obtain a permit to import the alcohol before releasing it to you. If you intend to bring back a substantial quantity of alcohol for your personal use, you should contact the port through which you will be re-entering the country, and make prior arrangements for entering the alcohol into the United States.
Also, you should be aware that state laws might limit the amount of alcohol you can bring in without a license. If you arrive in a state that has limitations on the amount of alcohol you may bring in without a license, that state law will be enforced by CBP, even though it may be more restrictive than federal regulations. We recommend that you check with the state government before you go abroad about their limitations on quantities allowed for personal importation and additional state taxes that might apply.
In practice, me thinks it's a little more liberal