OS X Leopard On October 26th
No longer a rumor, its been officially announced directly from Apple. They are doing the same thing as with the iPhones, closing down shop and then re-opening at 6PM which is when they will start selling copies of the new OS.
There are over 300 new features including Stacks, a new Finder, Quick Look, and Spaces, a Linux like feature that allows you to have multiple desktop workspaces and allows you to easily switch between them. In yet another rightfully deserved stab at Microsoft, Steve is quoted as saying that "everyone gets the 'Ultimate' version" for just $129.
Other than the $129 single-user license, there is a family package for $199, it is intended for a single household, but has 5 licenses for the new OS X. Of course, if you bought your new, qualifying, Mac on or after October 1st you will get a free copy for just the cost of shipping through the Mac OS Up-To-Date deal.
There is a new server edition of Leopard being released too (Microsoft, you should really give this a try, releasing your new server software alongside your new OS). You will be able to get a 10-client version for $499 for an unlimited client edition for $999.
Apple to Ship Maco OS X Leopard on October 26 [via Apple]