I go through King's Cross station almost every day, and the scenes in the movie were actually filmed between platforms 4 and 5, since the real platforms 9 and 10 only serve regional trains, and are actually quite dull. But around the launch of the movie Warner Brothers put up signs all around platform 9 saying things like First Year Students Queue Here and No Broomstick Flying on the Platform and Owls Must Be Caged. It was cool and funny when they first did it, but it rapidly got out of hand when they plastered every available surface with it.

Incidentally, the description in the book is actually platforms 9 and 10 at Euston Station; JK Rowling confused the two, but was in Manchester while writing that part of the book, and couldn't check. That's why the book talks about walking into the barrier between the platforms, but the movie has them walking into a pillar.