Apple has seeded Mac OS X 10.5 (Leopard) Preview to Apple Developer Connection developers with Select ($500/yr) or Premier accounts ($3500/yr).
The Leopard version provided is reportedly the same build that was demonstrated at WWDC (Build 9A241). Apple provided one online update to Leopard developers which brings the final build to 9A241e.
Apple demonstrated Mac OS X 10.5 (Leopard) at WWDC 2006 and provides a preview of its features on their website.
Mac OS X 10.5 is not expected to be released until Spring of 2007.
