Thing is it WILL come out before the autumn though.
Apple aren't going to leave a 17 month gap between iMac releases.
Also Apple aren't going to want to miss out on students buying for uni/college. If they leave it too late the competitors might eat into sales.
Also a lot more people are more tech savvy these days, especially when it comes to updates and specs. The quarterly report backs this up with mac sales having hardly any growth.
Apple also relies on word of mouth for their marketing buzz. I've told at least 10 people to not buy any mac since they are going to be updated soon.