depends on when you buy it, if its a about a week before the launch they might do it. I got my g5 2 weeks before they released the latest version and i tried to get it and they said no. The only time i managed to get it was when i got my powerbook 2 days before the launch and they included a Install CD of the new system with the purchase. I think you would have to go the apple store to do that though.
My advice wait untill they release it to be sure so you don't accidentally have to pay 115 2 days latter to get the new system.