Leopard isn't due till next year. The WWDC will show a preview of Leopard but the actual shipping date will, most likely, not be announced till later this year or early next year. Leopard will undergo a lot of refinement and changes between this preview and it's final release - Tiger changed quite considerably between it's preview at WWDC 2004 and release in April 2005 - Apple will be giving out a Developer's Beta of Leopard to facilitate this process.
From the day it is available in stores all new Macs purchased after that date will come with 10.5 either pre-installed or as a drop in DVD.
Apple has also offered the drop in DVD to users who purchased a Mac within a specified date range before the new OS begins shipping (about 2 weeks if I recall correctly).
Current Tiger users and those who purchase a new Mac between now and about 2 weeks before 10.5 goes on sale will have to purchase Leopard.
Apple has also offered the drop in DVD to users who purchased a Mac within a specified date range before the new OS begins shipping (about 2 weeks if I recall correctly).