http://usa.visa.com/media/business/ecomm_merch_guide.pdf

Page 83, last paragraph:

Card Verification Value 2 (CVV2) - A three-digit value that is printed on the back of a Visa card, provides a cryptographic check of the information embossed on a card, and assures the merchant, Acquirer, and Issuer that the card is valid.  The CVV2 is housed in the signature panel immediately after the full account number or the last four digits of the account number. CNP merchants should ask the customer
for the CVV2 to verify the card's authenticity.

For information security purposes, merchants are prohibited from storing CVV2 data.