My take--as an UN-informed NON-developer

is:
* The feature has not been "removed"--it was announced for the future since before Tiger, but Apple never said when exactly. I suspect the Leopard DOES have the feature since even Tiger has it (for testing purposes). But it was never listed as a Leopard feature. That was only ever a good guess.
* But I think it will remain a developer feature, not exposed to users, for a while. Apple won't claim it as a feature or talk about it publicly until it is ready to be provide a quality experience.
* Maybe 10.6, but I predict Apple will pull the lever some time next year via free update. Possibly alongside new displays, although I'd love the feature NOW on my current screens.
* So why the wait? Maybe the feature is truly not done, or more likely, a lot of apps don't play nice with it. Until most apps--especially the big names--play nice with res-independence, it will be a frustration or annoyance to users (at best) or a source of crashes and screen scrambling (at worst). And that means Apple's not wholly in control of when the feature is ready for the public. I think it will be available once the experience is a very consistent one.