Twenty years ago, the image of burning copies of Salman Rushdie's The Satanic Verses held aloft by thousand-strong mobs of protestors became an internationally familiar symbol of anger and offence. Kenan Malik examines how the Rushdie affair transformed the debate worldwide on multiculturalism, tolerance and free speech, helped fuel the rise of radical Islam and pointed the way to the horrors of 9/11 and 7/7.