Some have speculated that the big day will be Oct. 26th. I think a Saturday release is just as likely as a Friday afternoon release, so that makes Oct. 20th and 27th likely candidates in my book.
But there's another rumor tied to the launch of Leopard, one about the iPhone. The argument goes that the iPhone uses Leopard, so Apple can't launch an SDK until Leopard's released.
Well, it's easy enough to verify that in fact, the iPhone is running Leopard:
# uname -a
Darwin Mercury 9.0.0d1 Darwin Kernel Version 9.0.0d1: Fri Jun 22 00:38:56 PDT 2007; root:xnu-918.104.22.168-obj~1/RELEASE_ARM_S5L8900XRB iPhone1.1 Darwin
From what I can gather, this is remarkably similar to Leopard beta versions that released about the same time as the iPhone.
Meanwhile, my OS X 10.4 (Panther) laptop reports:
% uname -a
Darwin QuickSilver.local 8.10.1 Darwin Kernel Version 8.10.1: Wed May 23 16:33:00 PDT 2007; root:xnu-792.22.5~1/RELEASE_I386 i386 i386
Yet, all this proves is that if Apple were going to launch an SDK, and if the lack of a Leopard launch would prevent them from launching said SDK (which doesn't necessarily follow), that their hands would be tied until Leopard launches.
It doesn't support any argument that Apple is interested nor likely to actually release said SDK.
Even if Apple did release an SDK, I suspect that they would keep certain aspects of the iPhone locked down... which means that I'll be staying on version 1.0.2 because quite frankly having a jailbroken iPhone on version 1.0.2 is more useful than the three useful features that you get with 1.1.1 (namely, these are Bluetooth audio streaming, video out, and iTMS access).