32bit is on its way out the door, at least as far as Microsoft is concerned. With the launch of Vista and Windows Server 2008 on its way, Microsoft is committing to have these iterations of Windows be the last that will support 32bit CPUs. Now, while we're not expecting a new version of Windows anytime soon, the company is already planning ahead. They have at least eight new versions of Windows planned for the future, a all of which will require 64-bit processors:

Speaking during a morning keynote address WinHEC, Laing described the company as being "in the middle of a transition to 64-bit computing. Post-2008, we will transition to 64-bit. Many Microsoft products are becoming 64-bit only today, because they're realizing the benefits of 64-bit computing."
Now, this doesn't necessarily mean that 32bit software will be kicked to the curb as well. Just as 64-bit editions of Windows have the ability to run 32bit code, they likely will include some form of backwards compatibility, just as you can still run many Windows 3.11 apps on XP today. Time will tell, however.