Check the full article for more context-setting and projections
64-bit Windows may seem like a far-out techie luxury to most of us, but Microsoft already has quite a lot of experience with it: There have already been two 64-bit versions of Windows desktop and server. Remember, most of the core of the OS and much of the higher-level bits are common to both, and 64-bit Windows Server is a heavily used operating system. In fact, Microsoft has already announced that Windows Server 2008 will be the last Windows Server version with a 32-bit edition. When the mainstream desktop moves to 64 bits, it will, in many ways, be Version 3 or 4 of Windows 64.
Nothing ever seems to substitute for a mass public test, but it's not like they will be rookies at it. And according to the same Flores blog, the number and percentage of 64-bit Vista systems are jumping substantially worldwide, and even more domestically.