Forget Vista. Everyone else has. Windows XP 64-bit and Windows 2003 Server R2 64-bit will be the foils for Leopard and Leopard Server, and at a core level. Microsoft is frankly in good shape. It has assembled a quorum of 64-bit device drivers and is up to speed with install-time support for most AMD and Intel 64-bit x86 CPUs, and for homegrown and 3rd-party chipsets. ISVs (independent software vendors) are slow to climb aboard, as they always are, but all key hardware vendors have bought in and those all-important grassroots users are on board. Leopard will enter a market where Microsoft has an established 64-bit installed base regardless of Vista's status.
That Leopard will land in the middle of a substantial installed base of 64-bit Windows seats is actually a boon to Apple, because it doesn't have to waste days and dollars to educate buyers on the basic virtues of 64-bitness and virtualization and just-in-time compilation and other things Leopardian. Instead, Apple can do what it's always done: Take for granted that prospective buyers have sufficient knowledge of Windows and PCs. With one magical line of ad copy, Apple can assert, convincingly, that since Tiger already bests XP and Vista and rivals Win2K3 Server, Leopard being the better of Tiger leaves Windows well behind.
Apple has established a brilliant new marketing strategy: Apple only needs to compete with its own prior best. Flat lack of mention of competing x86 systems relieves Apple of the burden of competing for hardware mind share with Dell, IBM, HP and such. AMD learned that it's nearly impossible to market against a competitor that won't even acknowledge your existence.
And Apple's got another trick up its sleeve...