Switch To a Mac has an interesting theory on why Leopard will be released on May 11, 2007.
I don’t buy it, but I think the research involved is sound, and the theory has much more weight than “an anonymous source told us” that most of the sites post these days.
I still don’t believe Leopard will ship before WWDC because, as far as I’ve seen, the thing just isn’t ready yet. I think June is the earliest we’re going to see Leopard come into our homes, and anything sooner would probably be a mistake on Apple’s part (unless they have a stable and updated looking version of Leopard running in a secret bunker somewhere…which is entirely possible).
Still…this is a good speculation piece, and worth reading if you like that sort of thing…and you do, or you wouldn’t be checking out Apple blogs all the time 
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I still think it will be released before WWDC, WWDC is all about Leopard I know, but it seems like they would want people to be prepared before they came rather than just spring it all on them at the same time, besides I still think a portion of the WWDC will be about the extra iPhone features as its very close to its launch and the launch of new hardware (laptops, ect) I doubt there will be enough time to do it all and a complete breakdown of the new OS, hell they spent like 2 hours talking bout a phone
Friday, May 25th 2007. I’ve done the same research earlier this month as the website mentioned, but I also took into consideration the developer releases of each of the versions of Mac OS X prior to Leopard. And I noticed that ALL major versions have been released at the end of a month, between the 24th and the 29th. So May 25th it is. You can tell everybody menneke said so.