To echo bmillan01, top business schools generally want to see at least a 4 on the AWA. Scores lower than 4 will make admissions committees wonder about your ability to write at the graduate level. This is especially true when your application essays are polished but your AWA score is low - committees might wonder if you received help on your application.
Of course, the higher the score, the better, but schools are not looking for the next Hemingway here: they simply want to see that you can write a persuasive essay on an assigned topic. While it is not likely that a high AWA will convince a committee to accept you if the rest of your application is borderline, it is possible that a low AWA score will convince a committee to reject you if the rest of your application is not totally up to snuff.
Remember that business schools receive so many applications that they need to weed people out fairly quickly in the admissions process. Scoring poorly on the AWA can give schools a reason to say no.
Best of luck!
Matt Mapplebeck
Student Services Associate
Manhattan GMAT