From arstechnica, courtesy of NACDL news scan. In part:

A federal appeals court ruled Wednesday that law enforcement can legally scan or swipe a seized credit card—in fact, it is not a Fourth Amendment search at all, so it doesn’t require a warrant.

In the 8th Circuit Court of Appeals’ 15-page opinion, swiping a card does not constitute a physical search, as the magnetic stripe simply contains the same information obviously visible on the front of the card. Plus, the defendant, Eric-Arnaud Benjamin Briere De L’Isle, couldn’t have had a reasonable privacy interest in the card, the court concluded, because he would have tried to use it when he tried to buy something, thereby giving up privacy interests to a third party (the issuing bank).