Digitimes claims that Leopard may be delayed until October. Why? To include Vista support:
The sources pointed out that the launch delay is not due to software design problems with Leopard but instead is attributed to Apple's plan to have its new OS support Windows Vista through an integrated version of Boot Camp.
It should be noted that Digitimes has been notoriously inaccurate with their reports in the past, but a number of recent Digitimes rumors have been corroborated, if not confirmed. (LED-backlit Laptops, Apple Notebook with Robson Caching)