I think it will be late may to give developers time to see final version before wwdc
I don't see why Apple would need to do that. Developers have been testing Leopard since last August, and even if there is something truly different in 10.5 (like an enhaced GUI), the core of the new OS has been tested and re-tested by developers. No, I think Apple will announce it on June 11th, not just because it's my birthday, but because they will want to make as much noise as possible about it.
It won't be released without being announced in advance. Apple has done it that way for ever OS X release so far, with a big countdown counter on their main Web page. It would be great to see it announced at NAB with a May release, though.Well I personally can't see it being released at WWDC considering they will have other stuff to announce then (like the new MacBook Pros based on Intel's new chipset).
If it doesn't get released at NAB in a week, then I will go for May.