I think that iWork and iLife may have some sort of Leopard Special Goodness and they can't show them off without showing off leopard first. Bunches of Indie Mac developers are doing leopard only next versions (Delicious Monster, Textmate, etc...) Some for core animation, some for garbage collection in Obj C 2.0. Apple all but started this trend by abandoning panther with Safari 2.0 (and I'm sure there are other examples.)