IMO Leopard will be previewed around the end of April/ start of May and shipping in the last weeks of June.
Yes, I honestly believe that if Apple managed to keep the iPhone a secret for two years I surely believe they can hold those top-secret features for as much as they want. I don't think they are holding those features to prevent Microsoft from copying them, I think it's all related to the fact that Steve Jobs is a demanding person and whatever ideas he and his team came up with are being a programmer's hell and they're taking a while to be sorted out.
Regarding the fact that "Apple doesn't have any new features because if they did they would have shown'em to developers because of correct implementation"... ok, honestly, I don't understand crap about FreeBSD, Darwin, Unix or whatever Programming Language OSX runs on, however, from what I've learned so far (from coding HTML, XML and ActionScript) when the code is made correctly and well structured you minimize the risks involved in bugs and other problems. It also makes implementation a lot easier and dynamic.
So, I can only assume some of these new-features will simply be a layer of the OS that doesn't mess with the stuff that developers should have in their hands and I trust Apple programmers to know what they're doing.
One of the things that makes me strongly believe that these new-features will be there is the fact that Apple lives up to it's own credibility, they say, they do. Steve Jobs is a unique CEO in the world today and one of the things that makes him different than anyone else is the fact that what he says people can take for granted, thus the hype generated by the iPod, iPhone and other Apple products, everyone trusts his word. Can you imagine the tech-world headlines if Leopard is bundled exactly how it was shown in WWDC 06? I can:
"Steve LIED" : "Leopard Flop, Nothing top-secret about that" : "Leopard = Tiger" and bla bla bla.
What I'm saying is, they have to give us something, Steve said it, they can't take the risk of everyone pointing the finger on them for promising something as big as this and never talking about it again. Also, the first goal for Apple right now it to blow Vista away, and they're not going to impress the global PC population if they don't throw some fantastic features and more eye-candy into OSX right?

My 2 cents and sorry for the long reply.. I got carried away.