no idea as to when the new iMacs will begin shipping but historically Apple does not give coupons on new machines for an upcoming os. the current os is quite acceptable to run and many people don't want to deal with whatever bugs a new os may have anyway. now given that Apple has delayed Leopard from spring to October it is not without possibility that they would discount Leopard to new iMac owners, but still doubtful. What's the difference between someone who buys a new iMac after the announcement and someone who buys a new laptop after the iMac announcement. If they did this for the iMac, then they would have to do it for all their machines.
Reality is, if you really want a new iMac and Leopard, pay for both, or wait for Leopard to come out before you get your iMac.