Cory Doctorow has a piece in the U.K. Guardian Unlimited on a "delicious irony" he finds in a pop art exhibition currently up at London's National Portrait Gallery:
"Apparently [the artists whose work is featured in the exhibition] cut up magazines, copied comic books, drew trademarked cartoon characters like Minnie Mouse, reproduced covers from Time magazine, made ironic use of a cartoon Charles Atlas, painted over iconic photos of James Dean and Elvis Presley - and that's just in the first of seven rooms. ... Celebrated pop artists including Larry Poons, Robert Rauschenberg and Andy Warhol created these images by nicking the work of others, without permission, and transforming it to make statements and evoke emotions never countenanced by the original creators. Despite this, the programme does not say a word about copyright. ... Reading the programme, you can only assume that the curators' message about copyright is that where free expression is concerned, the rights of the creators of the original source material must take a back seat to those of the pop artists. There is, however, another message about copyright in the National Portrait Gallery: it is implicit in the 'No Photography' signs prominently displayed throughout its rooms .... These signs are not intended to protect the works from the depredations of camera flashes (otherwise they would read 'No Flash Photography'). No, the ban on pictures is meant to safeguard the copyright of the works hung on the walls - a fact that every member of staff I asked instantly confirmed. ... I wasn't even allowed to photograph the 'No Photographs' sign. A member of staff explained that the typography and layout of the signs was itself copyrighted."
Some New York museums follow this practice of not allowing any photography. Others, however, including MoMA and the Met, do allow it, with certain restrictions (for example, no flash). See here.