Amazon has begun offering Mac OS X 10.5 (Leopard) for pre-ordering on their website.
Amazon first offered Leopard for presale on June 12th, a day after Apple's WWDC keynote announcing Leopard's new features. It is already ranked as the 51st most popular software item on Amazon (as of this writing).
Mac OS X 10.5 Leopard is scheduled to ship in October 2007.
Update: Leopard is now ranked #1 amongst best selling software.
Update 2: Leopard Family Pack also available.