Definitely a prank. But the whole point of Banksy's career is to blur the lines between art and prank. And really, a found urinal that Duchamp autographed with someone else's name? That isn't even original at this point? That's not a prank? :-)
The movie definitely grows in value when you realize there's a huge real-life prank going on, and that no-one is quite sure exactly what it is. Kind of like not being sure if you're on a seedy street or in an art gallery. But that's a big part of the feeling that street art is supposed to give you, as nearly as I can tell, and Banksy and Fairey are at the top of the game at it.
For me, one of the big mysteries is who actually made the movie. You mean to tell me that Banksy and Shepard Fairey made that? It's a very well-done movie. It sets off new and old footage really well. It's engaging and provocative. So they do street art very well, and are really good at making films? Seems to much to hope for, but it certainly could be true. That's part of the fun, I think. :-)