I know that many Windows manufacturers offered free upgrades from Win-XP to Win-Vista for buyers purchasing a PC in the 3 months before Vista was released.
Wonder if Apple normally does that for new OS releases?
ETA: Also, I think they did that because MS kept missing target dates and PC makers were worried they wouldn't sell any computers before Christmas as people held out. I doubt the holdouts for leopard are quite so great (although I'm sure there are a bunch here).