Apple's annual developer conference in San Francisco.
Mac OS X 10.5 Leopard 'Finalized'? (Update: Not GM Yet)
The last publicly available Mac OS X 10.5 Leopard seed (9A559) was released to developers on September 21st, 2007. There is no word whether or not this seed represents the "Gold Master".
Apple has previously announced that Leopard will be shipping in October, but no official release date has yet been set. Circulating rumors have pegged October 26th as the likely date for the launch.
Update: Since the story was originally published, we've heard from a couple of sources that internal Apple builds are still progressing and "Gold Master" has not yet been declared. I suspect we'll hear more in the next few days.