Apple has officially announced October 26, 2007 as the shipping date for Mac OS X 10.5, a.k.a. Leopard. If you are interested in upgrading, you can place a pre-order at the online store (http://www.apple.com), but, as always, I recommend that you wait at least a month or two until others have shaken out the major bugs.
In the past, new Macs that were purchased after a new OS was released either came with the new software installed, with an additional set of disks so you can install it yourself, or with a coupon so you can get a free copy. If you are in the market for a new Mac, wait until after the release date and be sure to inquire about Leopard when you purchase.
For more information about the features in the new OS, see: