It's not taught in public school to stop subvocalizing (sounding out what you read in your head). I believe this is the barrier that keeps most people reading like elementary school kids.
I stopped subvocalizing while reading about two years ago in my senior year of highschool. My reading speed was around 300 WPM then. After a year of not subvocalizing I had a reading speed of 500 WPM with 70% comprehension. According to this test it's up to 700 WPM now, but there's no comprehension test, making it a somewhat useless measure.
Critics will argue there's no scientific-evidence that backs reading without subvocalization. However, I'm reading faster now and don't really care if there's proof backing that speed or not. I know a pink elephant when I see a pink elephant.
Steve made a recommendation for
PhotoReading. I've never cared enough to shuffle out $300 to take my reading speed further, but you might be interested in checking it out.