Everyone, even Microsoft, is openly talking about Windows 7. I think that there will be a 32-bit edition of Windows 7, but I also think it may be the last such version of Windows, And I wouldn't be surprised if Microsoft takes steps in it to encourage adoption of 64-bit Windows, moving us further on the path to a Windows 8, which may be entirely 64-bit.
There's a rule about Windows that most people never seem to appreciate fully: Every new version of Windows is designed for the next generation of hardware. When users upgrade XP to Vista on circa-2004 hardware and declare it sucky, they miss the point. This same effect is true of other operating systems to varying degrees; certainly it's true of MacOS, where they get to coordinate even more tightly. It's less true, I suppose, with Unix and variants because there's less of a partnership with hardware vendors.
A related important point about the cross-generational period is that upgrades just don't work as well as most preloaded installations. OEMs, at least in theory, can put a lot of work into getting their preloaded Windows and drivers to work optimally, while a user upgrading a previous version is likely to encounter tasks, identifying not only what should be running on the system but what should not, which they will have difficulty performing optimally.
The transition period, in which the mainstream shifts from 32-bit to 64-bit, is fraught with peril for Microsoft and users. There are many ways it can go badly. Microsoft has already tipped one of them in this blog by Windows exec Chris Flores. Essentially, it's the same hardware transition problem I just mentioned: Users trying to do an upgrade themselves may lack the technical experience and the detailed information about the hardware. He strongly recommends buying a preloaded system. OEMs aren't perfect at this; consider this blog in which Microsoft Distinguished Engineer Michael Fortin gives one example of suboptimal configuration by an OEM, although the data is limited to system boot time performance.
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.
ZDNet's Ed Bott extrapolates from Flores' numbers that "... at least 20% of all Vista PCs sold in the second quarter of this year came with 64-bit editions of Windows Vista pre-installed. By fall, it's possible, even likely, that we'll reach a tipping point, with more than 50% of new PCs sold at retail coming with 64-bit editions of Windows Vista pre-installed."