The US Customs booklet "Know Before You Go" lists duty free personal allowance of up to 100 cigars. See:
http://www.cbp.gov/xp/cgov/travel/leavingreturntoUS/vacation/know_brochure/dutyfree_exemption.xml

Cuban tobacco products are generally prohibited.
See 1998 Cuban Cigar Update (nothing newer): http://www.treas.gov/offices/eotffc/ofac/sanctions/ccigars.pdf.

This is from the Customs FAQ:
Can I import Cuban Cigars into the U.S.?

Persons returning either directly or indirectly from Cuba after a State Department-licensed visit are permitted to bring Cuban cigars into the United States. The value of the cigars may not exceed $100 U.S. dollars and they must be for the individual's personal use, not for resale. All other importations of Cuban cigars are illegal, including Cuban cigars that were acquired in other countries (such as Canada, England, or Mexico). It is also illegal for U.S. residents to buy, sell, trade, or otherwise engage in transactions involving illegally imported Cuban cigars. The penalties for doing so include confiscation of the cigars, civil fines of up to $55,000 per violation and in appropriate cases, criminal prosecution, which may result in higher fines and/or imprisonment. Foreign residents and visitors to the U.S.(i.e., French, Mexican etc) may not bring in goods of Cuban origin under any circumstances.