The excellent programme for Pop Art Portraits, the current exhibition at London's National Portrait Gallery, has a lot to say about the pictures hanging on the walls and the diverse source material the artists used to produce their provocative works.

Apparently they 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.

The programme describes the aesthetic experience conjured up by these transmogrified icons of high and low culture. 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. Can you blame the authors? A treatise on the way that copyright and trademarks were - had to be - trammelled to make these works could fill volumes.

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, including one by the entrance to the Pop Art Portraits exhibition.

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.

Indeed, it seems every square centimetre of the National Portrait Gallery is under some form of copyright. 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.

If true, presumably the same rules would prevent anyone from taking any pictures in any public place - unless you could somehow contrive to get a shot of Leicester Square without any writing, logos, architectural facades or images in it. Otherwise I doubt even Warhol could have got away with it.

So what's the message of the show? Is it a celebration of remix culture, revelling in the endless possibilities opened up by appropriating and reusing images without permission?

Or is it the epitaph on the tombstone of the sweet days before the UN set up the World Intellectual Property Organization and the ensuing mania for turning everything that can be sensed and recorded into someone's property?

Does this show - paid for with public money, with some works that are themselves owned by public institutions - seek to inspire us to become 21st century pop artists, armed with cameraphones, websites and mixers, or is it supposed to inform us that our chance has passed and we'd best settle for a life as information serfs who can't even make free use of what our eyes see and our ears hear?

Perhaps, just perhaps, this is actually a Dadaist show masquerading as a pop art show. Perhaps the point is to titillate us with the delicious irony of celebrating copyright infringement while simultaneously taking the view that even the "No Photography" sign is a form of property not to be reproduced without the permission that can never be had.

· Cory Doctorow is a digital activist, science fiction author and co-editor of the popular blog Boing Boing.