here is my theory, that I thought of (if someone else posted this, sorry, I didn't read it...)
My theory, is that if apple released leopard in late October, but not the updated MacBook, everyone waiting for leopard to make the purchase of there MacBook, will then purchase there MB, then a few weeks later (at most a month), they update the MacBooks... you know how mad that would make people? and then if they did this, not many will buy the new macbooks, because most people already bought them when leopard was released...
If they don't update leopard with the announcement\release of leopard... They will at least have to say they are updating them, or something... because that would be a major letdown....