Levitt and Dubner discuss gift cards in their Freakonomics column in today's New York Times magazine:

As for gift cards — well, let’s just say there is good reason that they
are known within the retail industry as a stored-value product: they
store their value very well, and often permanently. The
financial-services research firm TowerGroup estimates that of the $80
billion spent on gift cards in 2006, roughly $8 billion will never be
redeemed — “a bigger impact on consumers,” Tower notes, “than the
combined total of both debit- and credit-card fraud.” A survey by
Marketing Workshop Inc. found that only 30 percent of recipients use a
gift card within a month of receiving it, while Consumer Reports
estimates that 19 percent of the people who received a gift card in
2005 never used it.

Freakonomics
The Gift-Card Economy: Best Buys $16 Million Windfall
By STEPHEN J. DUBNER and STEVEN D. LEVITT
Published: January 7, 2007
The New York Times Magazine