I have worn 2 week disposables ever since I was 13, and have ALWAYS slept in them. My optician said you could halve the shelf-life of them and wear them 24 hours (so you can use 2 week lenses for 1 week if you never take them out, or one month lenses for two weeks, etc).
I never had any problems, but then a new kind of lense was developed about 18 months ago called "Focus Night and Day" which are designed to be worn 24 hours a day for one month at a time. I've been using them for just over a year now and they are great, and my optician is much happier with me now, because although she said the other way was okay, it wasn't ideal. These new lenses are supposed to allow 10 times more oxygen to your eyes than normal soft contacts.
I wonder if some of the previous posts about having contacts surgically removed from your cornea referred to hard contacts? Those you can definately not sleep in.