For all we know the feature complete version at WWDC will really be complete. Just, Apple won't start selling it to the public until October to keep it from removing too much hype from the iPhone. So in reality, Leopard may have been "on track." And, if I remember correctly, that was an Apple spokesperson who stated to some analyst that it was on track, not an Apple PR release. Correct me if I'm wrong, though.
Sorry but feature complete means all the features are present, it does not mean t will not panic every 5 minutes.
You are assuming it be perfect and ready for broad use and that they are just holding it up.
I would not make assumptions until we see what they deliver at the conference.