Native Instruments has a page about Kontakt 2 operability with the new Leopard OS.
Please see Native Instruments' official statement here:
"Native Instruments welcomes the improvements in Mac OS X 10.5, but does currently not recommend upgrading to the new operating system version for professional audio production or live performance."
"Installation -the installers of some NI products will currently not work under Mac OS X 10.5, causing the installation process to fail. This may also include previous versions of Service Center included with the product."
Operation - in some NI products, calling up certain message windows will cause the application to become unresponsive under Mac OS X 10.5.