I think we'll probably be seeing a WWDC release - it's the only window Apple's got to get up and actually talk about the secret features. There's no point having an event three weeks before WWDC or something - and they can't release it and then have Jobs come in two weeks later and unveil the secret features, because they wouldn't be secrets any more (people would find them, trust).
My money's on the iPhone seriously relying on Leopard to pull off some of the crazy secret stuff that's supposed to be coming. That's what will make Leopard a compelling upgrade - and it'll also be a point for people thinking about switching. Yeah, the iPhone's cool and all, and of course it works with your PC, but look how much cooler it is running on a Mac! You can stream your iTunes library to the phone, so you don't have to be at your computer to listen to your tunes! You can use the iPhone as a superwickedcool touch-screen visual remote for Front Row or your AppleTV! You can assign your Mac a lock and make your iPhone the key - just walk up to your Mac with your iPhone in your pocket, the lock's opened, and you're good to go (although to make sure you're not stuck if your iPhone breaks, you can also just type a password in)! All this and more!
It's that kind of thing that's going to peel people away from PCs - Vista's all well and good but there's upgrade woes, hassle with drivers and compatibility, the whole thing's a mess and I think stuff like the iPod and iPhone has presented people with a stepping stone, a gateway to a simpler way of using and living with their computer, of having their devices integrate seamlessly with little to zero effort on their part. That's the selling point of Macs, not ZFS Root or Spaces, which is what's lead me to think that the silence on Leopard is a red herring so large that not even a killer whale would try its luck with it. And I'd be surprised (and disappointed, frankly) if the iPhone didn't do at least one, if not all, of the ideas I've suggested, if not a hell of a lot more, when combined with Leopard, because that would shift a hell of a lot of iPhones. It would also shift a hell of a lot of new Macs, and copies of Leopard. I'm not sure the personal computer is ready for touchscreen as some people are suggesting, but I'm absolutely positive that Leopard's waiting on some hardware, and that hardware's got to be the iPhone.