Apple is probably finding it to be one of those projects that's easier to leave out of the final product than to overcome the complications of putting it in, plus few customers really miss it. But who knows, maybe it will still show up in some SL update.
It's sort of a 100:1 feature. Takes 100 times more manpower to create than something that would please the same number of users.
Like a new screen saver. Takes a week for one employee. 100 new customers. (Play along, please
Now RI: takes two years for five people. 500 new customers.