No, you shouldn't have any difficulty. They are treated like cash, lose em they're gone, give em away if you like. (As long as you are not talking about an iTunes or App Store card, those are locked once you use the code.)
US only, here is the excerpt from http://www.apple.com/legal/sales_policies/retail_us.html:
"Apple Gift Cards are issued and managed by Apple Value Services, LLC and can be applied only to purchases from an Apple Retail Store, the Apple Online Store, or the Apple Contact Center in the U.S. Gift Cards may not be redeemed at the iTunes Store, at any Apple resellers, for cash (except as required by law), or for shipments outside the U.S. Neither the issuer nor Apple is responsible for any lost or stolen gift cards or use without your permission. Keep them close; they’re valuable and can be redeemed by whoever finds them."